gistfile1.c

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);
    }
}