RFR: 8160327: Support for thumbnails present in APP1 marker for JPEG [v2]

[Phil Race]

On Fri, 14 Feb 2025 19:37:35 GMT, Brian Burkhalter <bpb at openjdk.org> wrote:

>> Jeremy has updated the pull request incrementally with one additional commit since the last revision:
>> 
>>   8160327: fixing typo so `thumbnailPos` can be zero
>>   
>>   The `thumbnailLength` cannot be zero, but the position can be.
>
> src/java.desktop/share/classes/com/sun/imageio/plugins/jpeg/JPEGImageReader.java line 1653:
> 
>> 1651:             if (exifMarkerSegment != null
>> 1652:                     && exifMarkerSegment.getNumThumbnails() == 1) {
>> 1653:                 return 1;
> 
> I know that my original code was also like this, but I think the eventual intent was to read both JFIF (APP0) and Exif (APP1) thumbnails if both are present. In such a case, the thumbnail count would be 2, the JFIF thumbnail would be at index 0, and the Exif thumbnail at index 1.
> 
> In general I would expect that if both of these thumbnails were present, then they would be identical. If this were not the case, however, then preferring the Exif thumbnail would be a behavioral change. This is not necessarily a blocker for the current PR, but it might need to be addressed later.

Hmm. I think it better to address it now. Who knows when we'd get back to it.

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PR Review Comment: https://git.openjdk.org/jdk/pull/22898#discussion_r1956750727