[Spice-devel] Wrong math in spice-common common/canvas_base.c ?

[Frediano Ziglio]

> 
> > 
> > Hi,
> >   was looking at usage of SPICE_UNALIGNED_CAST and I found this code:
> > 
> > 
> >     /* round down X to even 32 pixels (i.e. uint32_t) */
> >     extents.x1 = extents.x1 & ~(0x1f);
> > 
> >     mask_data_src = (uint8_t *)mask_data + mask_stride * extents.y1 +
> >     extents.x1 / 32;
> >     mask_data = SPICE_UNALIGNED_CAST(uint32_t *, mask_data_src);
> > 
> >     mask_x -= extents.x1;
> >     mask_y -= extents.y1;
> >     mask_width = extents.x2 - extents.x1;
> >     mask_height = extents.y2 - extents.y1;
> > 
> >     subimage = pixman_image_create_bits(PIXMAN_a1, mask_width, mask_height,
> >                                         mask_data, mask_stride);
> >     pixman_region32_init_from_image(&mask_region,
> >                                     subimage);
> >     pixman_image_unref(subimage);
> > 
> > 
> > the mask_data_src computation seems wrong to me however there seem no
> > reports
> > of rendering
> > issues. Particularly the "extents.x1 / 32" part.
> > The mask_data pointer is used to build a 1 bit image (PIXMAN_a1) used later
> > to initialize
> > a region (pixman_region32_init_from_image).
> > The initial mask_data (before mask_data_src computation) points to a
> > uint32_t
> > which is the
> > image raw data start. The fact that this data is used as a 1 bit image raw
> > data seems to
> > indicate that the image is 1 bit too. mask_stride is expressed in bytes (OT
> > and always
> > 4 multiple) and mask_data is cast to uint8_t* computing mask_data_src so it
> > seems that
> > the "extents.x1 / 32" offset should be in bytes however is is a pointer to
> > 1
> > bit raw data
> > should be "extents.x1 / 8".
> > 
> > Are we just lucky and issues never happens for some reasons (like very rare
> > rendering)
> > or the computation is wrong?
> > 
> 
> Tried to reproduce with some recording, I put an exit call in the code,
> apparently is a dead path, at least for my recordings!
> 

Ok, finally manage to get it.

First: Windows XP machine.
These commands are sent for the icons (or text under them, didn't understood) on
the desktop. To get an extends.x1 >= 32 you have to move an icon out of the
screen on the right side. As it have to be out of the screen quite a lot you
have to increase the icon size a bit (I set to 72, the maximum).
Now disable the grid alignment and move an icon out of the screen.
Select and unselect the icon.
Using "/ 32" the icon will have a white background instead of a transparent one
(probably due to white as default filler for the icon where transparent).
Using a "/ 8" (on the client is easier to see the difference) the icon is
rendered correctly.

Frediano