Bug 42972 – screen.colorDepth and screen.pixelDepth return 32 on Windows, but should return 24 (according to CSSOM View and Firefox)

• Component: DOM
• Status: RESOLVED FIXED

[Adam Roben (:aroben)]

To reproduce:

1. Load this URL: javascript:alert(screen.colorDepth + "\n" + screen.pixelDepth)

Both values are 32. But according to CSSOM View [1], they should be 24. Firefox returns 24. WebKit returns 24 on Mac, too.

[Adam Roben (:aroben)]

1. http://dev.w3.org/csswg/cssom-view/#dom-screen-colordepth

[Adam Roben (:aroben)]

It looks like we should fix this in the screenDepth function in PlatformScreenWin.cpp.

See <https://bugzilla.mozilla.org/show_bug.cgi?id=424386> and <https://bugzilla.mozilla.org/show_bug.cgi?id=466669> for Mozilla's discussion of this issue.

[Adam Roben (:aroben)]

(In reply to comment #2)
> It looks like we should fix this in the screenDepth function in PlatformScreenWin.cpp.

Here's the screenDepth function: <http://trac.webkit.org/browser/trunk/WebCore/platform/win/PlatformScreenWin.cpp?rev=52791#L68>.

The problem is that deviceInfo.dmBitsPerPel is 32 rather than 24 as we expect. We should notice this and return 24 instead.

[Adam Roben (:aroben)]

See also <http://dbaron.org/log/20100725-colorDepth>

[Adam Roben (:aroben)]

<rdar://problem/8234071>

[Adam Roben (:aroben)]

Created attachment 69249 [details]
Make sure screen.colorDepth/screen.pixelDepth don't include the bits used for alpha

[Adam Roben (:aroben)]

Comment on attachment 69249 [details]
Make sure screen.colorDepth/screen.pixelDepth don't include the bits used for alpha

View in context: https://bugs.webkit.org/attachment.cgi?id=69249&action=review

> LayoutTests/fast/dom/Window/window-screen-properties.html:6
> +if (depth == 16 || depth == 24) {
> +  document.write("PASS: window.screen.pixelDepth is either 16 or 24");

Hm, is 16 really an OK value? I guess that could mean 6+5+5 or something, but that seems strange.

[Adam Roben (:aroben)]

(In reply to comment #7)
> (From update of attachment 69249 [details])
> View in context: https://bugs.webkit.org/attachment.cgi?id=69249&action=review
> 
> > LayoutTests/fast/dom/Window/window-screen-properties.html:6
> > +if (depth == 16 || depth == 24) {
> > +  document.write("PASS: window.screen.pixelDepth is either 16 or 24");
> 
> Hm, is 16 really an OK value? I guess that could mean 6+5+5 or something, but that seems strange.

http://en.wikipedia.org/wiki/Color_depth#HighColor says that is exactly what happens (6 bits for green, 5 for red, 5 for blue). That page also implies we should be allowing many more values.

[Adam Roben (:aroben)]

Comment on attachment 69249 [details]
Make sure screen.colorDepth/screen.pixelDepth don't include the bits used for alpha

View in context: https://bugs.webkit.org/attachment.cgi?id=69249&action=review

>>> LayoutTests/fast/dom/Window/window-screen-properties.html:6
>>> +  document.write("PASS: window.screen.pixelDepth is either 16 or 24");
>> 
>> Hm, is 16 really an OK value? I guess that could mean 6+5+5 or something, but that seems strange.
> 
> http://en.wikipedia.org/wiki/Color_depth#HighColor says that is exactly what happens (6 bits for green, 5 for red, 5 for blue). That page also implies we should be allowing many more values.

Hyatt and I decided to change the test only to allow 24, since bug 42973 imposes that requirement, too.

[Adam Roben (:aroben)]

Committed r68808: <http://trac.webkit.org/changeset/68808>