Based on the PNGsuite.
(text copied from the official PNGsuite homepage)
Transparency should be used together with a background chunk. To test the combination of the two the latter 4 tests are there.
How to handle pictures with transparency, but without a background, opinions can differ.
Here we use black, but especially in the case of paletted images, the normal color would maybe even be better.
These images should all (except the 4th, 3rd & 2nd from the right) be rendered against the background color of the MNG's BACK chunk,
which is white. The two rows should look identical. The top row is the reference snapshot.
If the rows are not identical, your software does not correctly handle transparency and/or background processing.
The following images were used (left to right):
tbbn1g04 - transparent, black background chunk
tbbn2c16 - transparent, blue background chunk
tbbn3p08 - transparent, black background chunk
tbgn2c16 - transparent, green background chunk
tbgn3p08 - transparent, light-gray background chunk
tbrn2c08 - transparent, red background chunk
tbwn1g16 - transparent, white background chunk
tbwn3p08 - transparent, white background chunk
tbyn3p08 - transparent, yellow background chunk
tp0n1g08 - not transparent for reference (logo on gray)
tp0n2c08 - not transparent for reference (logo on gray)
tp0n3p08 - not transparent for reference (logo on gray)
tp1n3p08 - transparent, but no background chunk