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  Strange PNg Alpha Mask Behaviour
Posted by Andreas Lachner on Mar-01-2013 16:57
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Hi,
we are considering to swap our existing image library to chartdirector.

I am running a code like this:

   DrawArea           *pDA;
   int                iPixelColor;

   pDA=new DrawArea();
   pDA->setSize(xNewSize,yNewSize,Chart::Transparent);
   pDA->setPaletteMode(2);
   pDA->setTransparentColor(-1);

   for (xloop=0;xloop<xNewSize;xloop++)
    for (yloop=0;yloop<yNewSize;yloop++)
    {
     iPixelColor=lpbOriginalRGB[yloop*xNewSize*3+(xloop*3)+2]+(lpbOriginalRGB[yloop*xNewSize*3+(xloop*3)+1]<<8)+(lpbOriginalRGB[yloop*xNewSize*3+(xloop*3)+0]<<16);

     if (lpbDataNew[yloop*xNewSize+xloop]==2)
      iPixelColor+=(0x00<<24);
     else
      iPixelColor+=(0xFE<<24);

     pDA->pixel(xloop,yNewSize-yloop-1,iPixelColor);
    }

   pDA->outPNG("alpha.png");
   delete pDA;
  }

Basically lpbOriginalRGB holds and image and lpbDataNew the result of a background segmentation.

If I set the maximum transparency to 0xFE all is fine. If I set it to 0xFF (which would be correct), I get strange artefacts in the border regions between the image and the background (see enclosed).

Would you have any idea what causes that or am I doing something wrong?

Thanks
0xfe Alpha.png
0xff Alpha.png

  Re: Strange PNg Alpha Mask Behaviour
Posted by Peter Kwan on Mar-02-2013 03:42
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Hi Andreas,

In ChartDirector, the fully transparent color is 0xFF000000 (which is Chart::Transparent). All other colors of the form 0xFFRRGGBB will be treated as "dynamic colors" (like gradients, wallpaper patterns, etc). See:

http://www.advsofteng.com/doc/cdcpp.htm#colorspec.htm

The reason we use such a system is so that you can handle dynamic colors and normal colors in the same way (as a 32-bit integer).

So for you case, instead of "iPixelColor+=(0xFE<<24);", you may use "iPixelColor = Chart::Transparent;".

You may be aware that calling DrawArea.pixel for each pixel is inefficient if there are a huge number of pixels. For example, if the image is 1000 x 1000, the method will be called 1 million times. A faster method is to prepare the image in memory as an integer array 1000000 in size, then use it to create a "pattern color" (see DrawArea.patternColor), and use the patternColor to draw a rectangle on the DrawArea. It should be like:


int myColor = d->patternColor(IntArray(myArrayOfColors, xNewSize * yNewSize), yNewSize);
d->rect(0, 0, d->getWidth() - 1, d->getHeight() - 1, myColor, myColor);


Hope this can help.

Regards
Peter Kwan