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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 13:09:58
Message-ID: 20130418130958.13667.46383 () vidsolbach ! de
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This is an automatically generated e-mail. To reply, visit:
http://git.reviewboard.kde.org/r/109998/#review31244
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This review has been submitted with commit d01c7f821a9b1f0415fbce668a24341aadd61eff \
by Bernd Buschinski to branch master.

- Commit Hook


On April 13, 2013, 3:34 p.m., Bernd Buschinski wrote:
> 
> -----------------------------------------------------------
> This is an automatically generated e-mail. To reply, visit:
> http://git.reviewboard.kde.org/r/109998/
> -----------------------------------------------------------
> 
> (Updated April 13, 2013, 3:34 p.m.)
> 
> 
> 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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 <pre style="white-space: pre-wrap; white-space: -moz-pre-wrap; white-space: \
-pre-wrap; white-space: -o-pre-wrap; word-wrap: break-word;">This review has been \
submitted with commit d01c7f821a9b1f0415fbce668a24341aadd61eff by Bernd Buschinski to \
branch master.</pre>  <br />









<p>- Commit</p>


<br />
<p>On April 13th, 2013, 3:34 p.m. UTC, Bernd Buschinski wrote:</p>








<table bgcolor="#fefadf" width="100%" cellspacing="0" cellpadding="8" \
style="background-image: \
url('http://git.reviewboard.kde.org/static/rb/images/review_request_box_top_bg.ab6f3b1072c9.png'); \
background-position: left top; background-repeat: repeat-x; border: 1px black \
solid;">  <tr>
  <td>

<div>Review request for kdelibs.</div>
<div>By Bernd Buschinski.</div>


<p style="color: grey;"><i>Updated April 13, 2013, 3:34 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&#39;t want to introduce a &quot;is set&quot; 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 \
&quot;only&quot; 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>
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</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>







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</table>








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