On 01/23/2013 01:43 AM, Allan Sandfeld Jensen wrote:
On Wednesday 23 January 2013, Balazs Kelemen wrote:
On 01/22/2013 05:14 PM, Zoltan Herczeg wrote:
Where in WebKit do you experience problems with color conversion?
As for me WebKit2 transmits BGRA images, which needs to be converted to
RGBA before it is uploaded to a texture on GLES 2.0. These conversions
seems computation heavy for certain animations, and I was wondered
whether do we really need to use BGRA here. It would be nice to invent
something to avoid that.
You explained the symptom but not the source of the problem. So, do you
know _why_ do we have BGRA before the texture upload? If this is a
software rendered image buffer, why can't we just use the appropriate
format when doing the software rendering? Anybody?
Because it is a 32bit buffer of ARGB values. On a little endian CPU like i386,
32bit ARGB is stored bytewise as BGRA. In Qt we always have 32bit ARGB values
because that is the internal color format of the Qt renderer (QRgb). In the
grand scheme of things it is probably easier up to upload BGRA textures than
trying to double the rendering paths of QPainter to support RGBA.
A quick little overview of what I am talking about:
ARGB format aka RGBA32 (32bit ordered)
As 32bit math 8bit big endian 8bit little endian
24-31bit: A 1.byte: A 1.byte B
16-23bit: R 2.byte. R 2.byte G
8-15bit: G 3.byte. G 3.byte R
0-7bit: B 4.byte. B 4.byte A
RGBA format aka RGBA8 (byte-ordered)
As byte format 32bit big endian 32bit little endian
1.byte: R 24-31bit: R 24-31bit: A
2.byte: G 16-23bit: G 16-23bit: B
3.byte: B 8-15bit: B 8-15bit: G
4.byte: A 0-7bit: A 0-7bit: R
Ofcourse the confusion can be avoided if RGBA8 is only accesed bytewise, and
ARGB only 32bit wise, but when uploading to textures or otherwise serializing
it we need to deal with the mess.
Thanks, that was useful to me. I did not know that the internal format
is byte order agnostic.
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