Since I'm waiting for a build I sat down to implement this.
But our image checksums are not checksums of the image files :(, but
rather checksums of the pixels stored in the image.
And Tony points out that our image checksumming is completely insane:
=====
// Fix the alpha. The expected PNGs on Mac have an alpha channel, so we want
// to keep it. On Windows, the alpha channel is wrong since text/form control
// drawing may have erased it in a few places. So on Windows we force it to
// opaque and also don't write the alpha channel for the reference. Linux
// doesn't have the wrong alpha like Windows, but we ignore it anyway.
#if defined(OS_WIN)
bool discard_transparency = true;
device->makeOpaque(0, 0, src_bmp.width(), src_bmp.height());
#elif defined(OS_LINUX)
bool discard_transparency = true;
#elif defined(OS_MACOSX)
bool discard_transparency = false;
#endif
// Compute MD5 sum. We should have done this before calling
// device->makeOpaque on Windows. Because we do it after the call, there are
// some images that are the pixel identical on windows and other platforms
// but have different MD5 sums. At this point, rebaselining all the windows
// tests is too much of a pain, so we just check in different baselines.
====
To be more clear, here's a table of the platforms and their behaviors.
O=opaque, T=transparent.
(Sorry for my ghetto proportionally-spaced table here.)
Win Mac Lin
cksum O T T
png O T O
I conclude that on Linux, you cannot go from the PNG file back to the
checksum in the presence of alpha.
Just for fun I played around a bit with commands like:
convert path/to/pngfile rgba:- | md5sum
and wasn't able to repro the checksums I'm seeing.
It looks ok from
convert path/to/pngfile rgba:- | xxd -g4
(the RGBA<->BGRA problem doesn't apply for this black and while png file...).
In summary: tears.
On Mon, Jun 22, 2009 at 3:20 AM, Dean McNamee<[email protected]> wrote:
>
> Last week I updated our DEPS to pull in a newer version of Skia. I
> was stumped at a few cases where the checked in PNG looked completely
> wrong, but yet it was passing on the buildbots. There was no way that
> image could have been the output.
>
> It just dawned on me today, but I haven't verified it. I can dig up
> my commit to verify it, but I'd say 99% sure this was the case.
>
> If the checksum is valid, we don't even go to the PNG. Therefor I
> believe we have a bunch of layout tests where the checked in PNG is
> completely wrong, but the checksum is right.
>
> I don't have the time right now, but it would be great if someone
> could write a script and clean this up.
>
> Thanks
> -- dean
>
> >
>
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