/* cairo - a vector graphics library with display and print output
 *
 * Copyright © 2009 Eric Anholt
 * Copyright © 2009 Chris Wilson
 * Copyright © 2005,2010 Red Hat, Inc
 * Copyright © 2011 Linaro Limited
 * Copyright © 2011 Samsung Electronics
 *
 * This library is free software; you can redistribute it and/or
 * modify it either under the terms of the GNU Lesser General Public
 * License version 2.1 as published by the Free Software Foundation
 * (the "LGPL") or, at your option, under the terms of the Mozilla
 * Public License Version 1.1 (the "MPL"). If you do not alter this
 * notice, a recipient may use your version of this file under either
 * the MPL or the LGPL.
 *
 * You should have received a copy of the LGPL along with this library
 * in the file COPYING-LGPL-2.1; if not, write to the Free Software
 * Foundation, Inc., 51 Franklin Street, Suite 500, Boston, MA 02110-1335, USA
 * You should have received a copy of the MPL along with this library
 * in the file COPYING-MPL-1.1
 *
 * The contents of this file are subject to the Mozilla Public License
 * Version 1.1 (the "License"); you may not use this file except in
 * compliance with the License. You may obtain a copy of the License at
 * http://www.mozilla.org/MPL/
 *
 * This software is distributed on an "AS IS" basis, WITHOUT WARRANTY
 * OF ANY KIND, either express or implied. See the LGPL or the MPL for
 * the specific language governing rights and limitations.
 *
 * The Original Code is the cairo graphics library.
 *
 * The Initial Developer of the Original Code is Red Hat, Inc.
 *
 * Contributor(s):
 *	Benjamin Otte <otte@gnome.org>
 *	Carl Worth <cworth@cworth.org>
 *	Chris Wilson <chris@chris-wilson.co.uk>
 *	Eric Anholt <eric@anholt.net>
 *	Alexandros Frantzis <alexandros.frantzis@linaro.org>
 *	Henry Song <hsong@sisa.samsung.com>
 *	Martin Robinson <mrobinson@igalia.com>
 */

#include "cairoint.h"

#include "cairo-gl-private.h"

#include "cairo-composite-rectangles-private.h"
#include "cairo-clip-private.h"
#include "cairo-error-private.h"
#include "cairo-image-surface-private.h"

cairo_int_status_t
_cairo_gl_composite_set_source (cairo_gl_composite_t *setup,
			        const cairo_pattern_t *pattern,
				const cairo_rectangle_int_t *sample,
				const cairo_rectangle_int_t *extents,
				cairo_bool_t use_texgen)
{
    _cairo_gl_operand_destroy (&setup->src);
    return _cairo_gl_operand_init (&setup->src, pattern, setup->dst,
				   sample, extents, use_texgen);
}

void
_cairo_gl_composite_set_source_operand (cairo_gl_composite_t *setup,
					const cairo_gl_operand_t *source)
{
    _cairo_gl_operand_destroy (&setup->src);
    _cairo_gl_operand_copy (&setup->src, source);
}

void
_cairo_gl_composite_set_solid_source (cairo_gl_composite_t *setup,
				      const cairo_color_t *color)
{
    _cairo_gl_operand_destroy (&setup->src);
    _cairo_gl_solid_operand_init (&setup->src, color);
}

cairo_int_status_t
_cairo_gl_composite_set_mask (cairo_gl_composite_t *setup,
			      const cairo_pattern_t *pattern,
			      const cairo_rectangle_int_t *sample,
			      const cairo_rectangle_int_t *extents,
			      cairo_bool_t use_texgen)
{
    _cairo_gl_operand_destroy (&setup->mask);
    if (pattern == NULL)
        return CAIRO_STATUS_SUCCESS;

    return _cairo_gl_operand_init (&setup->mask, pattern, setup->dst,
				   sample, extents, use_texgen);
}

void
_cairo_gl_composite_set_mask_operand (cairo_gl_composite_t *setup,
				      const cairo_gl_operand_t *mask)
{
    _cairo_gl_operand_destroy (&setup->mask);
    if (mask)
	_cairo_gl_operand_copy (&setup->mask, mask);
}

void
_cairo_gl_composite_set_spans (cairo_gl_composite_t *setup)
{
    setup->spans = TRUE;
}

void
_cairo_gl_composite_set_multisample (cairo_gl_composite_t *setup)
{
    setup->multisample = TRUE;
}

void
_cairo_gl_composite_set_clip_region (cairo_gl_composite_t *setup,
                                     cairo_region_t *clip_region)
{
    setup->clip_region = clip_region;
}

void
_cairo_gl_composite_set_clip (cairo_gl_composite_t *setup,
			      cairo_clip_t *clip)
{
    setup->clip = clip;
}

static void
_cairo_gl_composite_bind_to_shader (cairo_gl_context_t   *ctx,
				    cairo_gl_composite_t *setup)
{
    _cairo_gl_shader_bind_matrix4f(ctx, ctx->current_shader->mvp_location,
				   ctx->modelviewprojection_matrix);
    _cairo_gl_operand_bind_to_shader (ctx, &setup->src,  CAIRO_GL_TEX_SOURCE);
    _cairo_gl_operand_bind_to_shader (ctx, &setup->mask, CAIRO_GL_TEX_MASK);
}

static void
_cairo_gl_texture_set_filter (cairo_gl_context_t *ctx,
                              GLuint              target,
                              cairo_filter_t      filter)
{
    switch (filter) {
    case CAIRO_FILTER_FAST:
    case CAIRO_FILTER_NEAREST:
	glTexParameteri (target, GL_TEXTURE_MIN_FILTER, GL_NEAREST);
	glTexParameteri (target, GL_TEXTURE_MAG_FILTER, GL_NEAREST);
	break;
    case CAIRO_FILTER_GOOD:
    case CAIRO_FILTER_BEST:
    case CAIRO_FILTER_BILINEAR:
	glTexParameteri (target, GL_TEXTURE_MIN_FILTER, GL_LINEAR);
	glTexParameteri (target, GL_TEXTURE_MAG_FILTER, GL_LINEAR);
	break;
    default:
    case CAIRO_FILTER_GAUSSIAN:
	ASSERT_NOT_REACHED;
    }
}

static void
_cairo_gl_texture_set_extend (cairo_gl_context_t *ctx,
                              GLuint              target,
                              cairo_extend_t      extend)
{
    GLint wrap_mode;
    assert (! _cairo_gl_device_requires_power_of_two_textures (&ctx->base) ||
            (extend != CAIRO_EXTEND_REPEAT && extend != CAIRO_EXTEND_REFLECT));

    switch (extend) {
    case CAIRO_EXTEND_NONE:
	if (ctx->gl_flavor == CAIRO_GL_FLAVOR_ES2)
	    wrap_mode = GL_CLAMP_TO_EDGE;
	else
	    wrap_mode = GL_CLAMP_TO_BORDER;
	break;
    case CAIRO_EXTEND_PAD:
	wrap_mode = GL_CLAMP_TO_EDGE;
	break;
    case CAIRO_EXTEND_REPEAT:
	if (ctx->has_npot_repeat)
	    wrap_mode = GL_REPEAT;
	else
	    wrap_mode = GL_CLAMP_TO_EDGE;
	break;
    case CAIRO_EXTEND_REFLECT:
	if (ctx->has_npot_repeat)
	    wrap_mode = GL_MIRRORED_REPEAT;
	else
	    wrap_mode = GL_CLAMP_TO_EDGE;
	break;
    default:
	wrap_mode = 0;
    }

    if (likely (wrap_mode)) {
	glTexParameteri (target, GL_TEXTURE_WRAP_S, wrap_mode);
	glTexParameteri (target, GL_TEXTURE_WRAP_T, wrap_mode);
    }
}


static void
_cairo_gl_context_setup_operand (cairo_gl_context_t *ctx,
                                 cairo_gl_tex_t      tex_unit,
                                 cairo_gl_operand_t *operand,
                                 unsigned int        vertex_offset,
                                 cairo_bool_t        vertex_size_changed)
{
    cairo_gl_dispatch_t *dispatch = &ctx->dispatch;
    cairo_bool_t needs_setup;

    /* XXX: we need to do setup when switching from shaders
     * to no shaders (or back) */
    needs_setup = vertex_size_changed;
    needs_setup |= _cairo_gl_operand_needs_setup (&ctx->operands[tex_unit],
                                                 operand,
                                                 vertex_offset);

    if (needs_setup) {
        _cairo_gl_composite_flush (ctx);
        _cairo_gl_context_destroy_operand (ctx, tex_unit);
    }

    memcpy (&ctx->operands[tex_unit], operand, sizeof (cairo_gl_operand_t));
    ctx->operands[tex_unit].vertex_offset = vertex_offset;

    if (! needs_setup)
        return;

    switch (operand->type) {
    default:
    case CAIRO_GL_OPERAND_COUNT:
        ASSERT_NOT_REACHED;
    case CAIRO_GL_OPERAND_NONE:
        break;
        /* fall through */
    case CAIRO_GL_OPERAND_CONSTANT:
        break;
    case CAIRO_GL_OPERAND_TEXTURE:
        glActiveTexture (GL_TEXTURE0 + tex_unit);
        glBindTexture (ctx->tex_target, operand->texture.tex);
        _cairo_gl_texture_set_extend (ctx, ctx->tex_target,
                                      operand->texture.attributes.extend);
        _cairo_gl_texture_set_filter (ctx, ctx->tex_target,
                                      operand->texture.attributes.filter);

	if (! operand->texture.texgen) {
	    dispatch->VertexAttribPointer (CAIRO_GL_TEXCOORD0_ATTRIB_INDEX + tex_unit, 2,
					   GL_FLOAT, GL_FALSE, ctx->vertex_size,
					   ctx->vb + vertex_offset);
	    dispatch->EnableVertexAttribArray (CAIRO_GL_TEXCOORD0_ATTRIB_INDEX + tex_unit);
	}
        break;
    case CAIRO_GL_OPERAND_LINEAR_GRADIENT:
    case CAIRO_GL_OPERAND_RADIAL_GRADIENT_A0:
    case CAIRO_GL_OPERAND_RADIAL_GRADIENT_NONE:
    case CAIRO_GL_OPERAND_RADIAL_GRADIENT_EXT:
        glActiveTexture (GL_TEXTURE0 + tex_unit);
        glBindTexture (ctx->tex_target, operand->gradient.gradient->tex);
        _cairo_gl_texture_set_extend (ctx, ctx->tex_target, operand->gradient.extend);
        _cairo_gl_texture_set_filter (ctx, ctx->tex_target, CAIRO_FILTER_BILINEAR);

	if (! operand->gradient.texgen) {
	    dispatch->VertexAttribPointer (CAIRO_GL_TEXCOORD0_ATTRIB_INDEX + tex_unit, 2,
					   GL_FLOAT, GL_FALSE, ctx->vertex_size,
					   ctx->vb + vertex_offset);
	    dispatch->EnableVertexAttribArray (CAIRO_GL_TEXCOORD0_ATTRIB_INDEX + tex_unit);
	}
	break;
    }
}

static void
_cairo_gl_context_setup_spans (cairo_gl_context_t *ctx,
			       cairo_bool_t        spans_enabled,
			       unsigned int        vertex_size,
			       unsigned int        vertex_offset)
{
    cairo_gl_dispatch_t *dispatch = &ctx->dispatch;

    if (! spans_enabled) {
	dispatch->DisableVertexAttribArray (CAIRO_GL_COLOR_ATTRIB_INDEX);
	ctx->spans = FALSE;
	return;
    }

    dispatch->VertexAttribPointer (CAIRO_GL_COLOR_ATTRIB_INDEX, 4,
				   GL_UNSIGNED_BYTE, GL_TRUE, vertex_size,
				   ctx->vb + vertex_offset);
    dispatch->EnableVertexAttribArray (CAIRO_GL_COLOR_ATTRIB_INDEX);
    ctx->spans = TRUE;
}

void
_cairo_gl_context_destroy_operand (cairo_gl_context_t *ctx,
                                   cairo_gl_tex_t tex_unit)
{
    cairo_gl_dispatch_t *dispatch = &ctx->dispatch;

    if  (!_cairo_gl_context_is_flushed (ctx))
	_cairo_gl_composite_flush (ctx);

    switch (ctx->operands[tex_unit].type) {
    default:
    case CAIRO_GL_OPERAND_COUNT:
        ASSERT_NOT_REACHED;
    case CAIRO_GL_OPERAND_NONE:
        break;
        /* fall through */
    case CAIRO_GL_OPERAND_CONSTANT:
        break;
    case CAIRO_GL_OPERAND_TEXTURE:
        dispatch->DisableVertexAttribArray (CAIRO_GL_TEXCOORD0_ATTRIB_INDEX + tex_unit);
        break;
    case CAIRO_GL_OPERAND_LINEAR_GRADIENT:
    case CAIRO_GL_OPERAND_RADIAL_GRADIENT_A0:
    case CAIRO_GL_OPERAND_RADIAL_GRADIENT_NONE:
    case CAIRO_GL_OPERAND_RADIAL_GRADIENT_EXT:
        dispatch->DisableVertexAttribArray (CAIRO_GL_TEXCOORD0_ATTRIB_INDEX + tex_unit);
        break;
    }

    memset (&ctx->operands[tex_unit], 0, sizeof (cairo_gl_operand_t));
}

static void
_cairo_gl_set_operator (cairo_gl_context_t *ctx,
                        cairo_operator_t    op,
			cairo_bool_t        component_alpha)
{
    struct {
	GLenum src;
	GLenum dst;
    } blend_factors[] = {
	{ GL_ZERO, GL_ZERO }, /* Clear */
	{ GL_ONE, GL_ZERO }, /* Source */
	{ GL_ONE, GL_ONE_MINUS_SRC_ALPHA }, /* Over */
	{ GL_DST_ALPHA, GL_ZERO }, /* In */
	{ GL_ONE_MINUS_DST_ALPHA, GL_ZERO }, /* Out */
	{ GL_DST_ALPHA, GL_ONE_MINUS_SRC_ALPHA }, /* Atop */

	{ GL_ZERO, GL_ONE }, /* Dest */
	{ GL_ONE_MINUS_DST_ALPHA, GL_ONE }, /* DestOver */
	{ GL_ZERO, GL_SRC_ALPHA }, /* DestIn */
	{ GL_ZERO, GL_ONE_MINUS_SRC_ALPHA }, /* DestOut */
	{ GL_ONE_MINUS_DST_ALPHA, GL_SRC_ALPHA }, /* DestAtop */

	{ GL_ONE_MINUS_DST_ALPHA, GL_ONE_MINUS_SRC_ALPHA }, /* Xor */
	{ GL_ONE, GL_ONE }, /* Add */
    };
    GLenum src_factor, dst_factor;

    assert (op < ARRAY_LENGTH (blend_factors));
    /* different dst and component_alpha changes cause flushes elsewhere */
    if (ctx->current_operator != op)
        _cairo_gl_composite_flush (ctx);
    ctx->current_operator = op;

    src_factor = blend_factors[op].src;
    dst_factor = blend_factors[op].dst;

    /* Even when the user requests CAIRO_CONTENT_COLOR, we use GL_RGBA
     * due to texture filtering of GL_CLAMP_TO_BORDER.  So fix those
     * bits in that case.
     */
    if (ctx->current_target->base.content == CAIRO_CONTENT_COLOR) {
	if (src_factor == GL_ONE_MINUS_DST_ALPHA)
	    src_factor = GL_ZERO;
	if (src_factor == GL_DST_ALPHA)
	    src_factor = GL_ONE;
    }

    if (component_alpha) {
	if (dst_factor == GL_ONE_MINUS_SRC_ALPHA)
	    dst_factor = GL_ONE_MINUS_SRC_COLOR;
	if (dst_factor == GL_SRC_ALPHA)
	    dst_factor = GL_SRC_COLOR;
    }

    if (ctx->current_target->base.content == CAIRO_CONTENT_ALPHA) {
        glBlendFuncSeparate (GL_ZERO, GL_ZERO, src_factor, dst_factor);
    } else if (ctx->current_target->base.content == CAIRO_CONTENT_COLOR) {
        glBlendFuncSeparate (src_factor, dst_factor, GL_ONE, GL_ONE);
    } else {
        glBlendFunc (src_factor, dst_factor);
    }
}

static cairo_status_t
_cairo_gl_composite_begin_component_alpha  (cairo_gl_context_t *ctx,
                                            cairo_gl_composite_t *setup)
{
    cairo_gl_shader_t *pre_shader = NULL;
    cairo_status_t status;

    /* For CLEAR, cairo's rendering equation (quoting Owen's description in:
     * http://lists.cairographics.org/archives/cairo/2005-August/004992.html)
     * is:
     *     mask IN clip ? src OP dest : dest
     * or more simply:
     *     mask IN CLIP ? 0 : dest
     *
     * where the ternary operator A ? B : C is (A * B) + ((1 - A) * C).
     *
     * The model we use in _cairo_gl_set_operator() is Render's:
     *     src IN mask IN clip OP dest
     * which would boil down to:
     *     0 (bounded by the extents of the drawing).
     *
     * However, we can do a Render operation using an opaque source
     * and DEST_OUT to produce:
     *    1 IN mask IN clip DEST_OUT dest
     * which is
     *    mask IN clip ? 0 : dest
     */
    if (setup->op == CAIRO_OPERATOR_CLEAR) {
        _cairo_gl_solid_operand_init (&setup->src, CAIRO_COLOR_WHITE);
	setup->op = CAIRO_OPERATOR_DEST_OUT;
    }

    /*
     * implements component-alpha %CAIRO_OPERATOR_OVER using two passes of
     * the simpler operations %CAIRO_OPERATOR_DEST_OUT and %CAIRO_OPERATOR_ADD.
     *
     * From http://anholt.livejournal.com/32058.html:
     *
     * The trouble is that component-alpha rendering requires two different sources
     * for blending: one for the source value to the blender, which is the
     * per-channel multiplication of source and mask, and one for the source alpha
     * for multiplying with the destination channels, which is the multiplication
     * of the source channels by the mask alpha. So the equation for Over is:
     *
     * dst.A = src.A * mask.A + (1 - (src.A * mask.A)) * dst.A
     * dst.R = src.R * mask.R + (1 - (src.A * mask.R)) * dst.R
     * dst.G = src.G * mask.G + (1 - (src.A * mask.G)) * dst.G
     * dst.B = src.B * mask.B + (1 - (src.A * mask.B)) * dst.B
     *
     * But we can do some simpler operations, right? How about PictOpOutReverse,
     * which has a source factor of 0 and dest factor of (1 - source alpha). We
     * can get the source alpha value (srca.X = src.A * mask.X) out of the texture
     * blenders pretty easily. So we can do a component-alpha OutReverse, which
     * gets us:
     *
     * dst.A = 0 + (1 - (src.A * mask.A)) * dst.A
     * dst.R = 0 + (1 - (src.A * mask.R)) * dst.R
     * dst.G = 0 + (1 - (src.A * mask.G)) * dst.G
     * dst.B = 0 + (1 - (src.A * mask.B)) * dst.B
     *
     * OK. And if an op doesn't use the source alpha value for the destination
     * factor, then we can do the channel multiplication in the texture blenders
     * to get the source value, and ignore the source alpha that we wouldn't use.
     * We've supported this in the Radeon driver for a long time. An example would
     * be PictOpAdd, which does:
     *
     * dst.A = src.A * mask.A + dst.A
     * dst.R = src.R * mask.R + dst.R
     * dst.G = src.G * mask.G + dst.G
     * dst.B = src.B * mask.B + dst.B
     *
     * Hey, this looks good! If we do a PictOpOutReverse and then a PictOpAdd right
     * after it, we get:
     *
     * dst.A = src.A * mask.A + ((1 - (src.A * mask.A)) * dst.A)
     * dst.R = src.R * mask.R + ((1 - (src.A * mask.R)) * dst.R)
     * dst.G = src.G * mask.G + ((1 - (src.A * mask.G)) * dst.G)
     * dst.B = src.B * mask.B + ((1 - (src.A * mask.B)) * dst.B)
     *
     * This two-pass trickery could be avoided using a new GL extension that
     * lets two values come out of the shader and into the blend unit.
     */
    if (setup->op == CAIRO_OPERATOR_OVER) {
	setup->op = CAIRO_OPERATOR_ADD;
	status = _cairo_gl_get_shader_by_type (ctx,
                                               &setup->src,
                                               &setup->mask,
					       setup->spans,
                                               CAIRO_GL_SHADER_IN_CA_SOURCE_ALPHA,
                                               &pre_shader);
        if (unlikely (status))
            return status;
    }

    if (ctx->pre_shader != pre_shader)
        _cairo_gl_composite_flush (ctx);
    ctx->pre_shader = pre_shader;

    return CAIRO_STATUS_SUCCESS;
}

static void
_scissor_to_doubles (cairo_gl_surface_t	*surface,
		     double x1, double y1,
		     double x2, double y2)
{
    double height;

    height = y2 - y1;
    if (_cairo_gl_surface_is_texture (surface) == FALSE)
	y1 = surface->height - (y1 + height);
    glScissor (x1, y1, x2 - x1, height);
    glEnable (GL_SCISSOR_TEST);
}

void
_cairo_gl_scissor_to_rectangle (cairo_gl_surface_t *surface,
		       const cairo_rectangle_int_t *r)
{
    _scissor_to_doubles (surface, r->x, r->y, r->x+r->width, r->y+r->height);
}

static void
_scissor_to_box (cairo_gl_surface_t	*surface,
		 const cairo_box_t	*box)
{
    double x1, y1, x2, y2;
    _cairo_box_to_doubles (box, &x1, &y1, &x2, &y2);
    _scissor_to_doubles (surface, x1, y1, x2, y2);
}

static cairo_bool_t
_cairo_gl_composite_setup_vbo (cairo_gl_context_t *ctx,
			       unsigned int size_per_vertex)
{
    cairo_bool_t vertex_size_changed = ctx->vertex_size != size_per_vertex;
    if (vertex_size_changed) {
	ctx->vertex_size = size_per_vertex;
	_cairo_gl_composite_flush (ctx);
    }

    if (_cairo_gl_context_is_flushed (ctx)) {
	ctx->dispatch.VertexAttribPointer (CAIRO_GL_VERTEX_ATTRIB_INDEX, 2,
					   GL_FLOAT, GL_FALSE, size_per_vertex,
					   ctx->vb);
	ctx->dispatch.EnableVertexAttribArray (CAIRO_GL_VERTEX_ATTRIB_INDEX);
    }

    return vertex_size_changed;
}

static void
_disable_stencil_buffer (void)
{
    glDisable (GL_STENCIL_TEST);
    glDepthMask (GL_FALSE);
}

static cairo_int_status_t
_cairo_gl_composite_setup_painted_clipping (cairo_gl_composite_t *setup,
					    cairo_gl_context_t *ctx,
					    int vertex_size)
{
    cairo_int_status_t status = CAIRO_INT_STATUS_SUCCESS;

    cairo_gl_surface_t *dst = setup->dst;
    cairo_clip_t *clip = setup->clip;

    if (clip->num_boxes == 1 && clip->path == NULL) {
	_scissor_to_box (dst, &clip->boxes[0]);
	goto disable_stencil_buffer_and_return;
    }

    if (! _cairo_gl_ensure_stencil (ctx, setup->dst)) {
	status = CAIRO_INT_STATUS_UNSUPPORTED;
	goto disable_stencil_buffer_and_return;
    }

    /* We only want to clear the part of the stencil buffer
     * that we are about to use. It also does not hurt to
     * scissor around the painted clip. */
    _cairo_gl_scissor_to_rectangle (dst, _cairo_clip_get_extents (clip));

    /* The clip is not rectangular, so use the stencil buffer. */
    glDepthMask (GL_TRUE);
    glEnable (GL_STENCIL_TEST);

    /* Texture surfaces have private depth/stencil buffers, so we can
     * rely on any previous clip being cached there. */
    if (_cairo_gl_surface_is_texture (setup->dst)) {
	cairo_clip_t *old_clip = setup->dst->clip_on_stencil_buffer;
	if (_cairo_clip_equal (old_clip, setup->clip))
	    goto activate_stencil_buffer_and_return;

	if (old_clip) {
	    _cairo_clip_destroy (setup->dst->clip_on_stencil_buffer);
	}

	setup->dst->clip_on_stencil_buffer = _cairo_clip_copy (setup->clip);
    }

    glClearStencil (0);
    glClear (GL_STENCIL_BUFFER_BIT);

    glStencilOp (GL_REPLACE, GL_REPLACE, GL_REPLACE);
    glStencilFunc (GL_EQUAL, 1, 0xffffffff);
    glColorMask (0, 0, 0, 0);

    status = _cairo_gl_msaa_compositor_draw_clip (ctx, setup, clip);

    if (unlikely (status)) {
	glColorMask (1, 1, 1, 1);
	goto disable_stencil_buffer_and_return;
    }

    /* We want to only render to the stencil buffer, so draw everything now.
       Flushing also unbinds the VBO, which we want to rebind for regular
       drawing. */
    _cairo_gl_composite_flush (ctx);
    _cairo_gl_composite_setup_vbo (ctx, vertex_size);

activate_stencil_buffer_and_return:
    glColorMask (1, 1, 1, 1);
    glStencilOp (GL_KEEP, GL_KEEP, GL_KEEP);
    glStencilFunc (GL_EQUAL, 1, 0xffffffff);
    return CAIRO_INT_STATUS_SUCCESS;

disable_stencil_buffer_and_return:
    _disable_stencil_buffer ();
    return status;
}

static cairo_int_status_t
_cairo_gl_composite_setup_clipping (cairo_gl_composite_t *setup,
				    cairo_gl_context_t *ctx,
				    int vertex_size)
{
    cairo_bool_t clip_changing = TRUE;
    cairo_bool_t clip_region_changing = TRUE;

    if (! ctx->clip && ! setup->clip && ! setup->clip_region && ! ctx->clip_region)
	goto disable_all_clipping;

    clip_changing = ! _cairo_clip_equal (ctx->clip, setup->clip);
    clip_region_changing = ! cairo_region_equal (ctx->clip_region, setup->clip_region);
    if (! _cairo_gl_context_is_flushed (ctx) &&
	(clip_region_changing || clip_changing))
	_cairo_gl_composite_flush (ctx);

    assert (!setup->clip_region || !setup->clip);

    /* setup->clip is only used by the msaa compositor and setup->clip_region
     * only by the other compositors, so it's safe to wait to clean up obsolete
     * clips. */
    if (clip_region_changing) {
	cairo_region_destroy (ctx->clip_region);
	ctx->clip_region = cairo_region_reference (setup->clip_region);
    }
    if (clip_changing) {
	_cairo_clip_destroy (ctx->clip);
	ctx->clip = _cairo_clip_copy (setup->clip);
    }

    /* For clip regions, we scissor right before drawing. */
    if (setup->clip_region)
	goto disable_all_clipping;

    if (setup->clip)
	return _cairo_gl_composite_setup_painted_clipping (setup, ctx,
                                                           vertex_size);
disable_all_clipping:
    _disable_stencil_buffer ();
    glDisable (GL_SCISSOR_TEST);
    return CAIRO_INT_STATUS_SUCCESS;
}

cairo_status_t
_cairo_gl_set_operands_and_operator (cairo_gl_composite_t *setup,
				     cairo_gl_context_t *ctx)
{
    unsigned int dst_size, src_size, mask_size, vertex_size;
    cairo_status_t status;
    cairo_gl_shader_t *shader;
    cairo_bool_t component_alpha;
    cairo_bool_t vertex_size_changed;

    component_alpha =
	setup->mask.type == CAIRO_GL_OPERAND_TEXTURE &&
	setup->mask.texture.attributes.has_component_alpha;

    /* Do various magic for component alpha */
    if (component_alpha) {
	status = _cairo_gl_composite_begin_component_alpha (ctx, setup);
	if (unlikely (status))
	    return status;
     } else {
	if (ctx->pre_shader) {
	    _cairo_gl_composite_flush (ctx);
	    ctx->pre_shader = NULL;
	}
    }

    status = _cairo_gl_get_shader_by_type (ctx,
					   &setup->src,
					   &setup->mask,
					   setup->spans,
					   component_alpha ?
					   CAIRO_GL_SHADER_IN_CA_SOURCE :
					   CAIRO_GL_SHADER_IN_NORMAL,
                                           &shader);
    if (unlikely (status)) {
	ctx->pre_shader = NULL;
	return status;
    }
    if (ctx->current_shader != shader)
        _cairo_gl_composite_flush (ctx);

    status = CAIRO_STATUS_SUCCESS;

    dst_size = 2 * sizeof (GLfloat);
    src_size = _cairo_gl_operand_get_vertex_size (&setup->src);
    mask_size = _cairo_gl_operand_get_vertex_size (&setup->mask);
    vertex_size = dst_size + src_size + mask_size;

    if (setup->spans)
	vertex_size += sizeof (GLfloat);

    vertex_size_changed = _cairo_gl_composite_setup_vbo (ctx, vertex_size);

    _cairo_gl_context_setup_operand (ctx, CAIRO_GL_TEX_SOURCE, &setup->src, dst_size, vertex_size_changed);
    _cairo_gl_context_setup_operand (ctx, CAIRO_GL_TEX_MASK, &setup->mask, dst_size + src_size, vertex_size_changed);

    _cairo_gl_context_setup_spans (ctx, setup->spans, vertex_size,
				   dst_size + src_size + mask_size);

    _cairo_gl_set_operator (ctx, setup->op, component_alpha);

    if (_cairo_gl_context_is_flushed (ctx)) {
	if (ctx->pre_shader) {
	    _cairo_gl_set_shader (ctx, ctx->pre_shader);
	    _cairo_gl_composite_bind_to_shader (ctx, setup);
	}
	_cairo_gl_set_shader (ctx, shader);
	_cairo_gl_composite_bind_to_shader (ctx, setup);
    }

    return status;
}

cairo_status_t
_cairo_gl_composite_begin (cairo_gl_composite_t *setup,
			   cairo_gl_context_t **ctx_out)
{
    cairo_gl_context_t *ctx;
    cairo_status_t status;

    assert (setup->dst);

    status = _cairo_gl_context_acquire (setup->dst->base.device, &ctx);
    if (unlikely (status))
	return status;

    _cairo_gl_context_set_destination (ctx, setup->dst, setup->multisample);
    glEnable (GL_BLEND);

    status = _cairo_gl_set_operands_and_operator (setup, ctx);
    if (unlikely (status))
	goto FAIL;

    status = _cairo_gl_composite_setup_clipping (setup, ctx, ctx->vertex_size);
    if (unlikely (status))
	goto FAIL;

    *ctx_out = ctx;

FAIL:
    if (unlikely (status))
        status = _cairo_gl_context_release (ctx, status);

    return status;
}

static inline void
_cairo_gl_composite_draw_tristrip (cairo_gl_context_t *ctx)
{
    cairo_array_t* indices = &ctx->tristrip_indices;
    const unsigned short *indices_array = _cairo_array_index_const (indices, 0);

    if (ctx->pre_shader) {
	cairo_gl_shader_t *prev_shader = ctx->current_shader;

	_cairo_gl_set_shader (ctx, ctx->pre_shader);
	_cairo_gl_set_operator (ctx, CAIRO_OPERATOR_DEST_OUT, TRUE);
	glDrawElements (GL_TRIANGLE_STRIP, _cairo_array_num_elements (indices), GL_UNSIGNED_SHORT, indices_array);

	_cairo_gl_set_shader (ctx, prev_shader);
	_cairo_gl_set_operator (ctx, CAIRO_OPERATOR_ADD, TRUE);
    }

    glDrawElements (GL_TRIANGLE_STRIP, _cairo_array_num_elements (indices), GL_UNSIGNED_SHORT, indices_array);
    _cairo_array_truncate (indices, 0);
}

static inline void
_cairo_gl_composite_draw_triangles (cairo_gl_context_t *ctx,
				    unsigned int count)
{
    if (! ctx->pre_shader) {
        glDrawArrays (GL_TRIANGLES, 0, count);
    } else {
        cairo_gl_shader_t *prev_shader = ctx->current_shader;

        _cairo_gl_set_shader (ctx, ctx->pre_shader);
        _cairo_gl_set_operator (ctx, CAIRO_OPERATOR_DEST_OUT, TRUE);
        glDrawArrays (GL_TRIANGLES, 0, count);

        _cairo_gl_set_shader (ctx, prev_shader);
        _cairo_gl_set_operator (ctx, CAIRO_OPERATOR_ADD, TRUE);
        glDrawArrays (GL_TRIANGLES, 0, count);
    }
}

static void
_cairo_gl_composite_draw_triangles_with_clip_region (cairo_gl_context_t *ctx,
						     unsigned int count)
{
    int i, num_rectangles;

    if (!ctx->clip_region) {
	_cairo_gl_composite_draw_triangles (ctx, count);
	return;
    }

    num_rectangles = cairo_region_num_rectangles (ctx->clip_region);
    for (i = 0; i < num_rectangles; i++) {
	cairo_rectangle_int_t rect;

	cairo_region_get_rectangle (ctx->clip_region, i, &rect);

	_cairo_gl_scissor_to_rectangle (ctx->current_target, &rect);
	_cairo_gl_composite_draw_triangles (ctx, count);
    }
}

static void
_cairo_gl_composite_unmap_vertex_buffer (cairo_gl_context_t *ctx)
{
    ctx->vb_offset = 0;
}

void
_cairo_gl_composite_flush (cairo_gl_context_t *ctx)
{
    unsigned int count;
    int i;

    if (_cairo_gl_context_is_flushed (ctx))
        return;

    count = ctx->vb_offset / ctx->vertex_size;
    _cairo_gl_composite_unmap_vertex_buffer (ctx);

    if (ctx->primitive_type == CAIRO_GL_PRIMITIVE_TYPE_TRISTRIPS) {
	_cairo_gl_composite_draw_tristrip (ctx);
    } else {
	assert (ctx->primitive_type == CAIRO_GL_PRIMITIVE_TYPE_TRIANGLES);
	_cairo_gl_composite_draw_triangles_with_clip_region (ctx, count);
    }

    for (i = 0; i < ARRAY_LENGTH (ctx->glyph_cache); i++)
	_cairo_gl_glyph_cache_unlock (&ctx->glyph_cache[i]);
}

static void
_cairo_gl_composite_prepare_buffer (cairo_gl_context_t *ctx,
				    unsigned int n_vertices,
				    cairo_gl_primitive_type_t primitive_type)
{
    if (ctx->primitive_type != primitive_type) {
	_cairo_gl_composite_flush (ctx);
	ctx->primitive_type = primitive_type;
    }

    assert(ctx->vbo_size > 0);
    if (ctx->vb_offset + n_vertices * ctx->vertex_size > ctx->vbo_size)
	_cairo_gl_composite_flush (ctx);
}

static inline void
_cairo_gl_composite_emit_vertex (cairo_gl_context_t *ctx,
				 GLfloat x, GLfloat y)
{
    GLfloat *vb = (GLfloat *) (void *) &ctx->vb[ctx->vb_offset];

    *vb++ = x;
    *vb++ = y;

    _cairo_gl_operand_emit (&ctx->operands[CAIRO_GL_TEX_SOURCE], &vb, x, y);
    _cairo_gl_operand_emit (&ctx->operands[CAIRO_GL_TEX_MASK  ], &vb, x, y);

    ctx->vb_offset += ctx->vertex_size;
}

static inline void
_cairo_gl_composite_emit_alpha_vertex (cairo_gl_context_t *ctx,
				       GLfloat x, GLfloat y, uint8_t alpha)
{
    GLfloat *vb = (GLfloat *) (void *) &ctx->vb[ctx->vb_offset];
    union fi {
	float f;
	GLbyte bytes[4];
    } fi;

    *vb++ = x;
    *vb++ = y;

    _cairo_gl_operand_emit (&ctx->operands[CAIRO_GL_TEX_SOURCE], &vb, x, y);
    _cairo_gl_operand_emit (&ctx->operands[CAIRO_GL_TEX_MASK  ], &vb, x, y);

    fi.bytes[0] = 0;
    fi.bytes[1] = 0;
    fi.bytes[2] = 0;
    fi.bytes[3] = alpha;
    *vb++ = fi.f;

    ctx->vb_offset += ctx->vertex_size;
}

static void
_cairo_gl_composite_emit_point (cairo_gl_context_t	*ctx,
				const cairo_point_t	*point)
{
    _cairo_gl_composite_emit_vertex (ctx,
				     _cairo_fixed_to_double (point->x),
				     _cairo_fixed_to_double (point->y));
}

static void
_cairo_gl_composite_emit_rect (cairo_gl_context_t *ctx,
                               GLfloat x1, GLfloat y1,
                               GLfloat x2, GLfloat y2)
{
    _cairo_gl_composite_prepare_buffer (ctx, 6,
					CAIRO_GL_PRIMITIVE_TYPE_TRIANGLES);

    _cairo_gl_composite_emit_vertex (ctx, x1, y1);
    _cairo_gl_composite_emit_vertex (ctx, x2, y1);
    _cairo_gl_composite_emit_vertex (ctx, x1, y2);

    _cairo_gl_composite_emit_vertex (ctx, x2, y1);
    _cairo_gl_composite_emit_vertex (ctx, x2, y2);
    _cairo_gl_composite_emit_vertex (ctx, x1, y2);
}

cairo_gl_emit_rect_t
_cairo_gl_context_choose_emit_rect (cairo_gl_context_t *ctx)
{
    return _cairo_gl_composite_emit_rect;
}

void
_cairo_gl_context_emit_rect (cairo_gl_context_t *ctx,
			     GLfloat x1, GLfloat y1,
			     GLfloat x2, GLfloat y2)
{
    _cairo_gl_composite_emit_rect (ctx, x1, y1, x2, y2);
}

static void
_cairo_gl_composite_emit_span (cairo_gl_context_t *ctx,
                               GLfloat x1, GLfloat y1,
                               GLfloat x2, GLfloat y2,
                               uint8_t alpha)
{
    _cairo_gl_composite_prepare_buffer (ctx, 6,
					CAIRO_GL_PRIMITIVE_TYPE_TRIANGLES);

    _cairo_gl_composite_emit_alpha_vertex (ctx, x1, y1, alpha);
    _cairo_gl_composite_emit_alpha_vertex (ctx, x2, y1, alpha);
    _cairo_gl_composite_emit_alpha_vertex (ctx, x1, y2, alpha);

    _cairo_gl_composite_emit_alpha_vertex (ctx, x2, y1, alpha);
    _cairo_gl_composite_emit_alpha_vertex (ctx, x2, y2, alpha);
    _cairo_gl_composite_emit_alpha_vertex (ctx, x1, y2, alpha);
}

static void
_cairo_gl_composite_emit_solid_span (cairo_gl_context_t *ctx,
				     GLfloat x1, GLfloat y1,
				     GLfloat x2, GLfloat y2,
				     uint8_t alpha)
{
    GLfloat *v;
    union fi {
	float f;
	GLbyte bytes[4];
    } fi;

    _cairo_gl_composite_prepare_buffer (ctx, 6,
					CAIRO_GL_PRIMITIVE_TYPE_TRIANGLES);
    v = (GLfloat *) (void *) &ctx->vb[ctx->vb_offset];

    v[15] = v[ 6] = v[0] = x1;
    v[10] = v[ 4] = v[1] = y1;
    v[12] = v[ 9] = v[3] = x2;
    v[16] = v[13] = v[7] = y2;

    fi.bytes[0] = 0;
    fi.bytes[1] = 0;
    fi.bytes[2] = 0;
    fi.bytes[3] = alpha;
    v[17] =v[14] = v[11] = v[8] = v[5] = v[2] = fi.f;

    ctx->vb_offset += 6*3 * sizeof(GLfloat);
}

cairo_gl_emit_span_t
_cairo_gl_context_choose_emit_span (cairo_gl_context_t *ctx)
{
    if (ctx->operands[CAIRO_GL_TEX_MASK].type != CAIRO_GL_OPERAND_NONE) {
	    switch (ctx->operands[CAIRO_GL_TEX_MASK].type) {
	    default:
	    case CAIRO_GL_OPERAND_COUNT:
		    ASSERT_NOT_REACHED;
	    case CAIRO_GL_OPERAND_NONE:
	    case CAIRO_GL_OPERAND_CONSTANT:
		    break;

	    case CAIRO_GL_OPERAND_LINEAR_GRADIENT:
	    case CAIRO_GL_OPERAND_RADIAL_GRADIENT_A0:
	    case CAIRO_GL_OPERAND_RADIAL_GRADIENT_NONE:
	    case CAIRO_GL_OPERAND_RADIAL_GRADIENT_EXT:
		    if (!ctx->operands[CAIRO_GL_TEX_MASK].gradient.texgen)
			    return _cairo_gl_composite_emit_span;
		    break;

	    case CAIRO_GL_OPERAND_TEXTURE:
		    if (!ctx->operands[CAIRO_GL_TEX_MASK].texture.texgen)
			    return _cairo_gl_composite_emit_span;
		    break;
	    }
    }

    switch (ctx->operands[CAIRO_GL_TEX_SOURCE].type) {
    default:
    case CAIRO_GL_OPERAND_COUNT:
        ASSERT_NOT_REACHED;
    case CAIRO_GL_OPERAND_NONE:
    case CAIRO_GL_OPERAND_CONSTANT:
	break;

    case CAIRO_GL_OPERAND_LINEAR_GRADIENT:
    case CAIRO_GL_OPERAND_RADIAL_GRADIENT_A0:
    case CAIRO_GL_OPERAND_RADIAL_GRADIENT_NONE:
    case CAIRO_GL_OPERAND_RADIAL_GRADIENT_EXT:
	if (!ctx->operands[CAIRO_GL_TEX_SOURCE].gradient.texgen)
		return _cairo_gl_composite_emit_span;
	break;

    case CAIRO_GL_OPERAND_TEXTURE:
	if (!ctx->operands[CAIRO_GL_TEX_SOURCE].texture.texgen)
		return _cairo_gl_composite_emit_span;
    }

    return _cairo_gl_composite_emit_solid_span;
}

static inline void
_cairo_gl_composite_emit_glyph_vertex (cairo_gl_context_t *ctx,
				       GLfloat x, GLfloat y,
				       GLfloat glyph_x, GLfloat glyph_y)
{
    GLfloat *vb = (GLfloat *) (void *) &ctx->vb[ctx->vb_offset];

    *vb++ = x;
    *vb++ = y;

    _cairo_gl_operand_emit (&ctx->operands[CAIRO_GL_TEX_SOURCE], &vb, x, y);

    *vb++ = glyph_x;
    *vb++ = glyph_y;

    ctx->vb_offset += ctx->vertex_size;
}

static void
_cairo_gl_composite_emit_glyph (cairo_gl_context_t *ctx,
                                GLfloat x1, GLfloat y1,
                                GLfloat x2, GLfloat y2,
                                GLfloat glyph_x1, GLfloat glyph_y1,
                                GLfloat glyph_x2, GLfloat glyph_y2)
{
    _cairo_gl_composite_prepare_buffer (ctx, 6,
					CAIRO_GL_PRIMITIVE_TYPE_TRIANGLES);

    _cairo_gl_composite_emit_glyph_vertex (ctx, x1, y1, glyph_x1, glyph_y1);
    _cairo_gl_composite_emit_glyph_vertex (ctx, x2, y1, glyph_x2, glyph_y1);
    _cairo_gl_composite_emit_glyph_vertex (ctx, x1, y2, glyph_x1, glyph_y2);

    _cairo_gl_composite_emit_glyph_vertex (ctx, x2, y1, glyph_x2, glyph_y1);
    _cairo_gl_composite_emit_glyph_vertex (ctx, x2, y2, glyph_x2, glyph_y2);
    _cairo_gl_composite_emit_glyph_vertex (ctx, x1, y2, glyph_x1, glyph_y2);
}

static void
_cairo_gl_composite_emit_solid_glyph (cairo_gl_context_t *ctx,
				      GLfloat x1, GLfloat y1,
				      GLfloat x2, GLfloat y2,
				      GLfloat glyph_x1, GLfloat glyph_y1,
				      GLfloat glyph_x2, GLfloat glyph_y2)
{
    GLfloat *v;

    _cairo_gl_composite_prepare_buffer (ctx, 6,
					CAIRO_GL_PRIMITIVE_TYPE_TRIANGLES);

    v = (GLfloat *) (void *) &ctx->vb[ctx->vb_offset];

    v[20] = v[ 8] = v[0] = x1;
    v[13] = v[ 5] = v[1] = y1;
    v[22] = v[10] = v[2] = glyph_x1;
    v[15] = v[ 7] = v[3] = glyph_y1;

    v[16] = v[12] = v[4] = x2;
    v[18] = v[14] = v[6] = glyph_x2;

    v[21] = v[17] = v[ 9] = y2;
    v[23] = v[19] = v[11] = glyph_y2;

    ctx->vb_offset += 4 * 6 * sizeof (GLfloat);
}

cairo_gl_emit_glyph_t
_cairo_gl_context_choose_emit_glyph (cairo_gl_context_t *ctx)
{
    switch (ctx->operands[CAIRO_GL_TEX_SOURCE].type) {
    default:
    case CAIRO_GL_OPERAND_COUNT:
        ASSERT_NOT_REACHED;
    case CAIRO_GL_OPERAND_NONE:
    case CAIRO_GL_OPERAND_CONSTANT:
	return _cairo_gl_composite_emit_solid_glyph;

    case CAIRO_GL_OPERAND_LINEAR_GRADIENT:
    case CAIRO_GL_OPERAND_RADIAL_GRADIENT_A0:
    case CAIRO_GL_OPERAND_RADIAL_GRADIENT_NONE:
    case CAIRO_GL_OPERAND_RADIAL_GRADIENT_EXT:
    case CAIRO_GL_OPERAND_TEXTURE:
	return _cairo_gl_composite_emit_glyph;
    }
}

void
_cairo_gl_composite_fini (cairo_gl_composite_t *setup)
{
    _cairo_gl_operand_destroy (&setup->src);
    _cairo_gl_operand_destroy (&setup->mask);
}

cairo_status_t
_cairo_gl_composite_set_operator (cairo_gl_composite_t *setup,
				  cairo_operator_t op,
				  cairo_bool_t assume_component_alpha)
{
    if (assume_component_alpha) {
        if (op != CAIRO_OPERATOR_CLEAR &&
            op != CAIRO_OPERATOR_OVER &&
            op != CAIRO_OPERATOR_ADD)
            return UNSUPPORTED ("unsupported component alpha operator");
    } else {
        if (! _cairo_gl_operator_is_supported (op))
            return UNSUPPORTED ("unsupported operator");
    }

    setup->op = op;
    return CAIRO_STATUS_SUCCESS;
}

cairo_status_t
_cairo_gl_composite_init (cairo_gl_composite_t *setup,
                          cairo_operator_t op,
                          cairo_gl_surface_t *dst,
                          cairo_bool_t assume_component_alpha)
{
    cairo_status_t status;

    memset (setup, 0, sizeof (cairo_gl_composite_t));

    status = _cairo_gl_composite_set_operator (setup, op,
					       assume_component_alpha);
    if (status)
	return status;

    setup->dst = dst;
    setup->clip_region = dst->clip_region;

    return CAIRO_STATUS_SUCCESS;
}

static cairo_int_status_t
_cairo_gl_composite_append_vertex_indices (cairo_gl_context_t	*ctx,
					   int			 number_of_new_indices)
{
    cairo_int_status_t status = CAIRO_INT_STATUS_SUCCESS;
    cairo_array_t *indices = &ctx->tristrip_indices;
    int number_of_indices = _cairo_array_num_elements (indices);
    unsigned short current_vertex_index = 0;
    int i;

    assert (number_of_new_indices > 0);

    /* If any preexisting triangle triangle strip indices exist on this
       context, we insert a set of degenerate triangles from the last
       preexisting vertex to our first one. */
    if (number_of_indices > 0) {
	const unsigned short *indices_array = _cairo_array_index_const (indices, 0);
	current_vertex_index = indices_array[number_of_indices - 1];

	status = _cairo_array_append (indices, &current_vertex_index);
	if (unlikely (status))
	    return status;

	current_vertex_index++;
	status =_cairo_array_append (indices, &current_vertex_index);
	if (unlikely (status))
	    return status;
    }

    for (i = 0; i < number_of_new_indices; i++) {
	status = _cairo_array_append (indices, &current_vertex_index);
	current_vertex_index++;
	if (unlikely (status))
	    return status;
    }

    return CAIRO_STATUS_SUCCESS;
}

cairo_int_status_t
_cairo_gl_composite_emit_quad_as_tristrip (cairo_gl_context_t	*ctx,
					   cairo_gl_composite_t	*setup,
					   const cairo_point_t	quad[4])
{
    _cairo_gl_composite_prepare_buffer (ctx, 4,
					CAIRO_GL_PRIMITIVE_TYPE_TRISTRIPS);

    _cairo_gl_composite_emit_point (ctx, &quad[0]);
    _cairo_gl_composite_emit_point (ctx, &quad[1]);

    /* Cairo stores quad vertices in counter-clockwise order, but we need to
       emit them from top to bottom in the triangle strip, so we need to reverse
       the order of the last two vertices. */
    _cairo_gl_composite_emit_point (ctx, &quad[3]);
    _cairo_gl_composite_emit_point (ctx, &quad[2]);

    return _cairo_gl_composite_append_vertex_indices (ctx, 4);
}

cairo_int_status_t
_cairo_gl_composite_emit_triangle_as_tristrip (cairo_gl_context_t	*ctx,
					       cairo_gl_composite_t	*setup,
					       const cairo_point_t	 triangle[3])
{
    _cairo_gl_composite_prepare_buffer (ctx, 3,
					CAIRO_GL_PRIMITIVE_TYPE_TRISTRIPS);

    _cairo_gl_composite_emit_point (ctx, &triangle[0]);
    _cairo_gl_composite_emit_point (ctx, &triangle[1]);
    _cairo_gl_composite_emit_point (ctx, &triangle[2]);
    return _cairo_gl_composite_append_vertex_indices (ctx, 3);
}
