Hi! As it seems that my previous PNG metadata patches are going somewhere, and also that serious feature changes require a lot of time, I now want to discuss a thing I noticed when working on the PNG metadata.
Consider a PNG file with compressed text chunks. How would they be expressed in Metadata? standard metadata: compression="zip" png metadata zTXt: compressionMethod="deflate" png metadata iTXt: compressionFlag="TRUE" compressionMethod="0" All the time the underlying compression method is the same. I suggest the following changes: 1. add "deflate" to the list of allowed values for the compression attribute in the standard metadata format. Rationale: to me, "zip" and "deflate" are different things. This corresponds to the difference between ZipOutputStream and DeflaterOutputStream in java.util.zip. I don't know whether there is in fact any image format actually using "zip", or if the meaning is "deflate" in every instance. If so, the "zip" value might be replaced. 2. change the combination of compressionFlag and compressionMethod for iTXt chunks to a single compression attribute with enumerated values, including the values "none" and "deflate". Rationale: Having to deal with integer constants defined in the PNG specification is cumbersome and contrary to implementation elsewhere in the metadata formats. Using "none" as one of the values gets rid of the separate compressionFlag and thus avoids dealing with strange cases where compressionFlag is "FALSE" and the compressionMethod attrribute is missing, unallowed, or whatever. As both these changes break compatibility, I guess they both should be included in a version bump of the metadata formats from 1.0 to 1.1. Large parts of the metadata handling code would be unaffected by this change, only a few places would require case distinctions. If you agree with my opinions, I would start working on an appropriate patch. If you have different suggestions, let me know. If there is something else that would require a bump of metadata versions, now would be a good time to collect this information. Greetings, Martin von Gagern
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