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

[chnav]

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 (!!!)
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


Thanks and Regards
chnav


Monday, June 6, 2016, 9:52:54 PM LOCAL