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Based on the PNGsuite.
The following samples test the delta-image PROM function.
The top image shows what the following MNG sample should look like. If there are any discrepencies,
your MNG decoder does not handle the PROM chunk adequately.
This will always be the case for implementations based on libmng-1.0.4 or older.
In this sample the original images are non-interlaced and the 'left-bit-replication' option of the PROM chunk
is used for the promotion operation.
The first row of the sample shows the original images. For each subsequent row each original image
is 'promoted' to a higher level (where possible). So the second row shows the 1-bit grayscale input promoted
to 2-bit grayscale, the next row shows 1- & 2-bit grayscale images promoted to 4-bit grayscale, etc.
Impossible promotions are marked with a red cross.
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