Based on the PNGsuite.
The following samples test various delta-image variations on grayscale targets with an alpha channel.
In each sample the top row of images is displayed against a plain background to show the original image that
is used in delta-operations below it. A background for the remaining canvas is generated from
a grayscale+alpha image with respectively 8 or 16 bits bit_depth (subsequent samples).
For each a sample with a complete white and a complete black background are shown.
Below the original images, the first 2 rows shows a pixel-replace (4) and pixel-addition (1) operation respectively.
This is only possible for grayscale+alpha delta's (the last 2 original images).
The next two rows show a color-replace (5) and color-addition (2) operation respectively.
The bottom two rows show an alpha-replace (6) and alpha-addition (3) operation.
These last 4 rows are only possible with single-channel delta's (the first 5 original images).
The results will look bizar in most cases!
The left side of each sample shows a screenshot of what the right side should look like.
If they do not match, your MNG decoder doesn't handle delta-image variations very well.
Note that this will be the case for implementations using libmng-1.0.4 or older!