#9 OctreeQuantizer sometimes resizes images before quantizing

[Simon Bridewell]

The OctreeQuantizer is resizing some images whilst quantizing them. Although it keeps the original image size, it shrinks or grows the image whilst drawing the original image onto the quantized image, and if shrinking the image, it fills the remainder of the quantized image with transparent pixels.

[Simon Bridewell]

Original JPEG image

[Simon Bridewell]

Quantized image showing resizing effect

[Simon Bridewell]

I've added an example - a photo in JPEG format and the bitmap which OctreeQuantizer creates from it.

This bug seems to affect mainly photos which are loaded from file as 24 BPP RGB format.

[Simon Bridewell]

And the solution...

I spent the best part of 2 weeks trying to work out where I'd introduced this bug, before finally discovering that it was present in the original Octree quantizer code I'd downloaded from http://msdn.microsoft.com/en-us/library/aa479306.aspx

The Quantizer.Quantize method in the GifComponents.Tools namespace (it's in the ImageManipulation namespace in the original source code) contains the following:

using( Graphics g = Graphics.FromImage ( copy ) )
{
g.PageUnit = GraphicsUnit.Pixel;

// Draw the source image onto the copy bitmap,
// which will effect a widening as appropriate.
g.DrawImageUnscaled ( source, bounds );
}

Replace this with the following:

using( Graphics g = Graphics.FromImage ( copy ) )
{
// Draw the source image onto the copy bitmap,
// which will effect a widening as appropriate.
g.DrawImage( source, bounds, bounds, GraphicsUnit.Pixel );
}

This will ensure that the original will be drawn onto the copy correctly, even if they have different resolutions.

I'll commit this fix to the repository when I've finished changing all the quantizer types to share a common base class, but here it is if anyone wants to apply it to their local copies before then.

[Simon Bridewell]

Fix has been committed to the repository

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