The cLLi chunk adds static metadata which provides an opportunity to optimize tone mapping of the associated content to a specific target display. This is accomplished by tailoring the tone mapping of the content itself to the specific peak brightness capabilities of the target display to prevent clipping. The method of tone-mapping optimization is currently subjective.
If we are aware of implementations that do something with this, then we could check that they do different things with, say, one image reporting MaxFALL of 100 and MaxFall of 4000. But what they actually do is completely unconstrained.
For a PNG validator or checker, it could report the presence of the chunk, check that the sizer of the data are correct and present the values, but that is about all.
For content creation tools, a list of tools that writes the chunk is helpful to demonstrate implementation.
svgeesus
commented
1 week ago
For browsers and image viewers, there is no normative requirement that can be tested:
If we are aware of implementations that do something with this, then we could check that they do different things with, say, one image reporting MaxFALL of 100 and MaxFall of 4000. But what they actually do is completely unconstrained.
For a PNG validator or checker, it could report the presence of the chunk, check that the sizer of the data are correct and present the values, but that is about all.
For content creation tools, a list of tools that writes the chunk is helpful to demonstrate implementation.