[cairo] W32: Drawing globally-transparent stuff with cairo

LRN lrn1986 at gmail.com
Sun Apr 6 02:43:27 PDT 2014 

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On 30.03.2014 2:29, LRN wrote:
> On 27.03.2014 19:54, LRN wrote:
>> I've been trying to somehow hack GTK to have RGBA GdkWindows,
>> with mixed results so far (i either get massive alpha corruption,
>> or crash pixman inside bitblt).
> 
>> Decided to take a step back and see if i can make it work 100% 
>> correctly with just GDI & cairo. Turns out, i can't.
> 
>> I've attached an example, compile with gcc main.c -o main.exe 
>> -I/mingw/include/cairo -lcairo -lgdi32 -ldwmapi
> 
>> It draws two gradients - one transparent, and one opaque. 
>> Transparency only works when window is invalidated and repainted
>>  (such as when it is obscured by another window and later shown 
>> again).
> 
>> The problem is most likely W32-specific, and not directly related
>>  to cairo, but i simply can't find what is wrong :(
> 
> 
> I'd like to thank everyone for the help.
> 
> The problem was mostly in W32 display config (glass transparency
> not enabled).
> 
> I'm now convinced that the rest of my problems are due to a bug in 
> cairo/pixman.
> 
> GTK bug tracking this is: 
> https://bugzilla.gnome.org/show_bug.cgi?id=727316
> 
> 

Traced inaccessible memory to _cairo_image_surface_map_to_image(),
which gives pixman a pointer and claims that it points to image data
of *width*, *height* with a *stride* - which turns out to be false.

The patch that verifies that memory is accessible is attached.

Backtrace is attached.

- -- 
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-------------- next part --------------
--- cairo-bb17403622e9ea474017e0039af8ee48b2b285db/src/cairo-image-surface.c.orig	2014-03-21 13:10:19.000000000 +0000
+++ cairo-bb17403622e9ea474017e0039af8ee48b2b285db/src/cairo-image-surface.c	2014-04-06 09:37:15.850899100 +0000
@@ -809,10 +809,12 @@
     cairo_image_surface_t *other = abstract_other;
     cairo_surface_t *surface;
     uint8_t *data;
+    uint8_t tmp;
 
     data = other->data;
     data += extents->y * other->stride;
     data += extents->x * PIXMAN_FORMAT_BPP (other->pixman_format)/ 8;
+    tmp = data[other->stride * extents->height - 1];
 
     surface =
 	_cairo_image_surface_create_with_pixman_format (data,
-------------- next part --------------
Program received signal SIGSEGV, Segmentation fault.
0x68dec6d0 in _cairo_image_surface_map_to_image (abstract_other=0x333a290, extents=0x28f4e0) at ../../cairo-bb17403622e9ea474017e0039af8ee48b2b285db/src/cairo-image-surface.c:817
817         tmp = data[other->stride * extents->height - 1];
(gdb) bt
#0  0x68dec6d0 in _cairo_image_surface_map_to_image (abstract_other=0x333a290, extents=0x28f4e0) at ../../cairo-bb17403622e9ea474017e0039af8ee48b2b285db/src/cairo-image-surface.c:817
#1  0x68e29783 in _cairo_surface_map_to_image (surface=0x333a290, extents=0x28f4e0) at ../../cairo-bb17403622e9ea474017e0039af8ee48b2b285db/src/cairo-surface.c:644
#2  0x68e5da94 in _cairo_win32_display_surface_map_to_image (abstract_surface=0x333aa20, extents=0x28f4e0) at ../../cairo-bb17403622e9ea474017e0039af8ee48b2b285db/src/win32/cairo-win32-display-surface.c:478
#3  0x68e29783 in _cairo_surface_map_to_image (surface=0x333aa20, extents=0x28f4e0) at ../../cairo-bb17403622e9ea474017e0039af8ee48b2b285db/src/cairo-surface.c:644
#4  0x68ddbbd4 in _cairo_fallback_compositor_paint (_compositor=0x68ea8a08 <_cairo_fallback_compositor>, extents=0x28f498) at ../../cairo-bb17403622e9ea474017e0039af8ee48b2b285db/src/cairo-fallback-compositor.c:59
#5  0x68dd788d in _cairo_compositor_paint (compositor=0x68ea8a08 <_cairo_fallback_compositor>, surface=0x333aa20, op=CAIRO_OPERATOR_SOURCE, source=0x28f7d8, clip=0x3339ce8) at ../../cairo-bb17403622e9ea474017e0039af8ee48b2b285db/src/cairo-compositor.c:65
#6  0x68e5e3c1 in _cairo_win32_display_surface_paint (surface=0x333aa20, op=CAIRO_OPERATOR_SOURCE, source=0x28f7d8, clip=0x3339ce8) at ../../cairo-bb17403622e9ea474017e0039af8ee48b2b285db/src/win32/cairo-win32-display-surface.c:785
#7  0x68e2b10e in _cairo_surface_paint (surface=0x333aa20, op=CAIRO_OPERATOR_SOURCE, source=0x28f7d8, clip=0x3339ce8) at ../../cairo-bb17403622e9ea474017e0039af8ee48b2b285db/src/cairo-surface.c:2081
#8  0x68ddff8b in _cairo_gstate_paint (gstate=0x3337678) at ../../cairo-bb17403622e9ea474017e0039af8ee48b2b285db/src/cairo-gstate.c:1067
#9  0x68ddae34 in _cairo_default_context_paint (abstract_cr=0x3337658) at ../../cairo-bb17403622e9ea474017e0039af8ee48b2b285db/src/cairo-default-context.c:958
#10 0x68dd12cc in cairo_paint (cr=0x3337658) at ../../cairo-bb17403622e9ea474017e0039af8ee48b2b285db/src/cairo.c:2003
#11 0x70edd524 in gdk_window_end_paint (window=0x1d3e0b0) at ../../gtk+-3.12.0/gdk/gdkwindow.c:2864
#12 0x66552f6f in gtk_main_do_event (event=0x28fa50) at ../../gtk+-3.12.0/gtk/gtkmain.c:1648
#13 0x70ecac0f in _gdk_event_emit (event=event at entry=0x28fa50) at ../../gtk+-3.12.0/gdk/gdkevents.c:69
#14 0x70ed752c in _gdk_window_process_updates_recurse_helper (window=0x1d3e0b0, expose_region=<optimized out>, dx=<optimized out>, dy=0) at ../../gtk+-3.12.0/gdk/gdkwindow.c:3392
#15 0x70ed5958 in gdk_window_process_updates_internal (window=window at entry=0x1d3e0b0) at ../../gtk+-3.12.0/gdk/gdkwindow.c:3500
#16 0x70ed5a5b in gdk_window_process_updates_with_mode (window=<optimized out>, recurse_mode=<optimized out>) at ../../gtk+-3.12.0/gdk/gdkwindow.c:3697
#17 0x63a4637a in _g_closure_invoke_va (closure=0x1e0c3b8, return_value=0x0, instance=0x1d3e1f8, args=0x28fd18 "?AO\001x?3\003", n_params=0, param_types=0x0) at gclosure.c:831
#18 0x63a5eb8a in g_signal_emit_valist (instance=instance at entry=0x1d3e1f8, signal_id=signal_id at entry=153, detail=detail at entry=0, var_args=var_args at entry=0x28fd18 "?AO\001x?3\003") at gsignal.c:3215
#19 0x63a5fa7e in g_signal_emit_by_name (instance=0x1d3e1f8, detailed_signal=detailed_signal at entry=0x70f198f8 <__FUNCTION__.28014+264> "paint") at gsignal.c:3403
#20 0x70ecfb08 in gdk_frame_clock_paint_idle (data=0x1d3e1f8) at ../../gtk+-3.12.0/gdk/gdkframeclockidle.c:430
#21 0x70ec1595 in gdk_threads_dispatch (data=data at entry=0x1d8d340) at ../../gtk+-3.12.0/gdk/gdk.c:635
#22 0x685f2995 in g_timeout_dispatch (source=source at entry=0x3338c78, callback=0x70ec1550 <gdk_threads_dispatch>, user_data=0x1d8d340) at gmain.c:4473
#23 0x685f1dcf in g_main_dispatch (context=0x1d5b7c0) at gmain.c:3064
#24 g_main_context_dispatch (context=context at entry=0x1d5b7c0) at gmain.c:3663
#25 0x685f212b in g_main_context_iterate (context=<optimized out>, block=block at entry=1, dispatch=dispatch at entry=1, self=<optimized out>) at gmain.c:3734
#26 0x685f2557 in g_main_loop_run (loop=0x3337f48) at gmain.c:3928
#27 0x6655218d in gtk_main () at ../../gtk+-3.12.0/gtk/gtkmain.c:1192
#28 0x0040286e in main (argc=1, argv=0x799f50) at ./main.c:20

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