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gl: Fudge gradient color generation to handle multiple stops at 0

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In order to generate the correct left-hand border color, we need to
fudge the offsets of the color stops if multiple stops are defined at 0.
The reason is that pixman will generate our color ramp by using the
right-most color stop for the pixel centre, but in order to provide the
sample colour outside of the gradient we need pixel 0 to be have the
left-most color.

Reported by Henry Song.
This commit is contained in:
Chris Wilson 2012-08-11 17:55:49 +01:00
parent b0336e9aad
commit adfe9b7eb6
1 changed files with 29 additions and 0 deletions
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29
src/cairo-gl-gradient.c
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@ -83,6 +83,27 @@ _cairo_gl_gradient_sample_width (unsigned int n_stops,
return (width + 7) & -8;
}
static uint8_t premultiply(double c, double a)
{
int v = c * a * 256;
return v - (v >> 8);
}
static uint32_t color_stop_to_pixel(const cairo_gradient_stop_t *stop)
{
uint8_t a, r, g, b;
a = stop->color.alpha_short >> 8;
r = premultiply(stop->color.red, stop->color.alpha);
g = premultiply(stop->color.green, stop->color.alpha);
b = premultiply(stop->color.blue, stop->color.alpha);
if (_cairo_is_little_endian ())
return a << 24 | r << 16 | g << 8 | b << 0;
else
return a << 0 | r << 8 | g << 16 | b << 24;
}
static cairo_status_t
_cairo_gl_gradient_render (const cairo_gl_context_t *ctx,
unsigned int n_stops,
@ -156,6 +177,14 @@ _cairo_gl_gradient_render (const cairo_gl_context_t *ctx,
pixman_image_unref (gradient);
pixman_image_unref (image);
/* We need to fudge pixel 0 to hold the left-most color stop and not
* the neareset stop to the zeroth pixel centre in order to correctly
* populate the border color. For completeness, do both edges.
*/
((uint32_t*)bytes)[0] = color_stop_to_pixel(&stops[0]);
((uint32_t*)bytes)[width-1] = color_stop_to_pixel(&stops[n_stops-1]);
return CAIRO_STATUS_SUCCESS;
}