[cairo] BUG: CAIRO_FORMAT_ARGB32, image_surface transparency = CAIRO_IMAGE_IS_OPAQUE after write_png()

[Adrian Johnson]

On 07/06/16 05:37, chnav wrote:
> Hi,
> 
> have discovered sporatic bug working with transparent images,
> which I finally managed to reproduce.
> 
> ------- code 1 begin --------------
> cairo_surface_t *surface = cairo_image_surface_create (CAIRO_FORMAT_ARGB32, 256, 256);
> cairo_t *cr = cairo_create (surface);
> // Initialisation
> cairo_set_operator(cr, CAIRO_OPERATOR_SOURCE);
> cairo_set_source_rgba(cr, 1.0, 1.0, 1.0, 1.0);
> cairo_paint(cr);
> cairo_surface_write_to_png(surface, "cairo_test3a.png");
> // ...
> cairo_set_source_rgba(cr, 1.0, 1.0, 1.0, 0.5);
> cairo_paint(cr);
> cairo_surface_write_to_png(surface, "cairo_test3b.png");
> ------- code 1 end --------------
> 
> Both files supposed to be 32 bit PNG with alpha but above will produce
> 24 bit png without alpha. 
> 
> 
> ------- code 2 begin --------------
> cairo_surface_t *surface = cairo_image_surface_create (CAIRO_FORMAT_ARGB32, 256, 256);
> cairo_t *cr = cairo_create (surface);
> // Initialisation
> cairo_set_operator(cr, CAIRO_OPERATOR_SOURCE);
> cairo_set_source_rgba(cr, 1.0, 1.0, 1.0, 0.9);
> cairo_paint(cr);
> cairo_surface_write_to_png(surface, "cairo_test3a.png");
> // ...
> cairo_set_source_rgba(cr, 1.0, 1.0, 1.0, 0.5);
> cairo_paint(cr);
> cairo_surface_write_to_png(surface, "cairo_test3b.png");
> ------- code 2 end --------------
> 
> Now both files are 32 bit PNG alpha.
> 
> 
> Short explanation, tracing "code 1" in debugger:
> 
> 1. Before 1st call of write_png():
>    image->transparency = CAIRO_IMAGE_UNKNOWN;
> 2. cairo_surface_write_to_png() calls _cairo_image_analyze_transparency();
> 3. _cairo_image_analyze_transparency() see CAIRO_IMAGE_UNKNOWN and
>    scans every pixels' alpha. It founds all of them equal 255 and changes
>    image->transparency = CAIRO_IMAGE_IS_OPAQUE (!!!)

If all alpha is 255 the image is opaque.

> 4. From now on any subsequent call of _cairo_image_analyze_transparency()
>    will return CAIRO_IMAGE_IS_OPAQUE;
> 5. cairo_surface_write_to_png() sets png_color_type = PNG_COLOR_TYPE_RGB;
> 
> 
> I assume if we create image surface CAIRO_FORMAT_ARGB32 then
> image->transparency must be CAIRO_IMAGE_HAS_ALPHA

That assumption is incorrect. Cairo analyses image transparency to
ensure it uses the most efficient format when writing out images to PNG,
PDF, and PS.

> 
> 
> Thanks and Regards
> chnav
> 
> 
> Monday, June 6, 2016, 9:52:54 PM LOCAL
>