Public comments on this W3C Recommendation are welcome. Please send them to the archived list png-group@w3.org .
For the tRNS chunk, Colour type 2, a typographical error was introduced in the second edition of PNG: the blue and green channels were switched. The first edition of PNG had the correct order, Red, Green, Blue. This is the same order as is used throughout the specification, and implementations have silently corrected for the obvious error.
In section 11.3.2.1 tRNS Transparency change:
| Colour type 0 | |
|---|---|
| Grey sample value | 2 bytes |
| Colour type 2 | |
| Red sample value | 2 bytes |
| Blue sample value | 2 bytes |
| Green sample value | 2 bytes |
| Colour type 3 | |
| Alpha for palette index 0 | 1 byte |
| Alpha for palette index 1 | 1 byte |
| ...etc... | 1 byte |
to:
| Colour type 0 | |
|---|---|
| Grey sample value | 2 bytes |
| Colour type 2 | |
| Red sample value | 2 bytes |
| Green sample value | 2 bytes |
| Blue sample value | 2 bytes |
| Colour type 3 | |
| Alpha for palette index 0 | 1 byte |
| Alpha for palette index 1 | 1 byte |
| ...etc... | 1 byte |
The phrase "pixel size" is ambiguous as it could refer to the geometric extent of a pixel; in fact the number of bits is meant.
As agreed and further clarified , in Clause 4.4, PNG Image, change the text in []:
c. Truecolor: ... The alpha channel may be represented by
a single [pixel] value. Matching pixels ...
d. Greyscale: ... The alpha channel may be represented by
a single [pixel value as in] the previous case. ...
... size of each sample, not the total [pixel size].
to
c. Truecolor: ... The alpha channel may be represented by
a single RGB pixel value. Matching pixels ...
d. Greyscale: ... The alpha channel may be represented by
a single greyscale pixel value, similar to the previous case. ...
... size of each sample, not the total number of bits per pixel.
in 13.13 Decoder gamma handling, change the text:
When the incoming image has unknown gamma (gAMA, sRGB, and iCCP all absent), choose a likely default gamma value, but allow the user to select a new one if the result proves too dark or too light. The default gamma may depend on other knowledge about the image, for example whether it came from the Internet or from the local system.
to
When the incoming image has unknown gamma (gAMA, sRGB, and iCCP all absent), standalone image viewers should choose a likely default gamma value, but allow the user to select a new one if the result proves too dark or too light. The default gamma may depend on other knowledge about the image, for example whether it came from the Internet or from the local system. For consistency, viewers for document formats such as HTML, or vector graphics such as SVG, should treat embedded or linked PNG images with unknown gamma in the same way that they treat other untagged images.