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List: kde-core-devel
Subject: Re: Review Request 109998: kjs: Fix CanvasImageData eating the color value by doing wrong alpha prem
From: "Commit Hook" <null () kde ! org>
Date: 2013-04-18 13:10:11
Message-ID: 20130418131011.14188.53774 () vidsolbach ! de
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http://git.reviewboard.kde.org/r/109998/
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(Updated April 18, 2013, 1:10 p.m.)
Status
------
This change has been marked as submitted.
Review request for kdelibs.
Description
-------
previous knowledge:
- the CanvasImageData allows setting the single color red/green/blue/alpha-values for \
each pixel
- the default for all pixel is Rgba 0,0,0,0 and must not be changed
- you can set the color-rgba values in any order, there is no fixed order
the bug:
We use QImage::Format_ARGB32_Premultiplied beacuse it faster for many operations.
As we directly set the red/green/blue/alpha color value via QImage::scanLine,
we must premultiply it, to keep it in the QImage::Format_ARGB32_Premultiplied format.
But as CanvasImageData does not specify any order of setting the values or does not \
know what values are set, we might end up doing a wrong premultiply.
For premultiply we MUST have the correct alpha value, if it is not set it is assumed \
0 ( the default), but if we premultiply a given (for example) red value with 0, its \
get zero, the red color value is gone. Gone and no way to recover it.
So for example if we set for each pixel (in this order) \
red=255,green=0,blue=0,alpha=255, we get a black image instead of a red one.
my solution:
As I don't want to introduce a "is set" check for the alpha value, I suggest we use \
ARG32 instead of ARGB32_Premultiplied. This allows us to set the color values in any \
order we want.
possible problems:
ARG32 is slower than ARGB32_Premultiplied.
Yes, but as far as I can see we do not display the image directly, it is "only" used \
as in QPainter::setCompositionMode(QPainter::CompositionMode_Source);
QPainter::drawImage(x,y,ourImage);
to get painted on another image.
I hope QPainter uses some sse magic, so the paint is faster than our custom \
premultiply.
btw:
While (afaik) it is possible to re-use that not-displayed ImageData, I only found \
javascript code creating new ImageData for each animation-frame. In this case \
ARGB32_Premultiplied would be more performant.
Diffs
-----
khtml/html/html_canvasimpl.cpp c7fce27
Diff: http://git.reviewboard.kde.org/r/109998/diff/
Testing
-------
http://www.w3schools.com/tags/canvas_createimagedata.asp
shows the problem, as it shows a black image but it should be red.
Thanks,
Bernd Buschinski
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This is an automatically generated e-mail. To reply, visit:
<a href="http://git.reviewboard.kde.org/r/109998/">http://git.reviewboard.kde.org/r/109998/</a>
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<h1 style="margin-right: 0.2em; padding: 0; font-size: 10pt;">This change has been \
marked as submitted.</h1> </td>
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<div>Review request for kdelibs.</div>
<div>By Bernd Buschinski.</div>
<p style="color: grey;"><i>Updated April 18, 2013, 1:10 p.m.</i></p>
<h1 style="color: #575012; font-size: 10pt; margin-top: 1.5em;">Description </h1>
<table width="100%" bgcolor="#ffffff" cellspacing="0" cellpadding="10" \
style="border: 1px solid #b8b5a0"> <tr>
<td>
<pre style="margin: 0; padding: 0; white-space: pre-wrap; white-space: \
-moz-pre-wrap; white-space: -pre-wrap; white-space: -o-pre-wrap; word-wrap: \
break-word;">previous knowledge:
- the CanvasImageData allows setting the single color red/green/blue/alpha-values for \
each pixel
- the default for all pixel is Rgba 0,0,0,0 and must not be changed
- you can set the color-rgba values in any order, there is no fixed order
the bug:
We use QImage::Format_ARGB32_Premultiplied beacuse it faster for many operations.
As we directly set the red/green/blue/alpha color value via QImage::scanLine,
we must premultiply it, to keep it in the QImage::Format_ARGB32_Premultiplied format.
But as CanvasImageData does not specify any order of setting the values or does not \
know what values are set, we might end up doing a wrong premultiply.
For premultiply we MUST have the correct alpha value, if it is not set it is assumed \
0 ( the default), but if we premultiply a given (for example) red value with 0, its \
get zero, the red color value is gone. Gone and no way to recover it.
So for example if we set for each pixel (in this order) \
red=255,green=0,blue=0,alpha=255, we get a black image instead of a red one.
my solution:
As I don't want to introduce a "is set" check for the alpha value, I \
suggest we use ARG32 instead of ARGB32_Premultiplied. This allows us to set the color \
values in any order we want.
possible problems:
ARG32 is slower than ARGB32_Premultiplied.
Yes, but as far as I can see we do not display the image directly, it is \
"only" used as in \
QPainter::setCompositionMode(QPainter::CompositionMode_Source); \
QPainter::drawImage(x,y,ourImage); to get painted on another image.
I hope QPainter uses some sse magic, so the paint is faster than our custom \
premultiply.
btw:
While (afaik) it is possible to re-use that not-displayed ImageData, I only found \
javascript code creating new ImageData for each animation-frame. In this case \
ARGB32_Premultiplied would be more performant.
</pre>
</td>
</tr>
</table>
<h1 style="color: #575012; font-size: 10pt; margin-top: 1.5em;">Testing </h1>
<table width="100%" bgcolor="#ffffff" cellspacing="0" cellpadding="10" style="border: \
1px solid #b8b5a0"> <tr>
<td>
<pre style="margin: 0; padding: 0; white-space: pre-wrap; white-space: \
-moz-pre-wrap; white-space: -pre-wrap; white-space: -o-pre-wrap; word-wrap: \
break-word;">http://www.w3schools.com/tags/canvas_createimagedata.asp shows the \
problem, as it shows a black image but it should be red.</pre> </td>
</tr>
</table>
<h1 style="color: #575012; font-size: 10pt; margin-top: 1.5em;">Diffs</b> </h1>
<ul style="margin-left: 3em; padding-left: 0;">
<li>khtml/html/html_canvasimpl.cpp <span style="color: grey">(c7fce27)</span></li>
</ul>
<p><a href="http://git.reviewboard.kde.org/r/109998/diff/" style="margin-left: \
3em;">View Diff</a></p>
</td>
</tr>
</table>
</div>
</body>
</html>
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