Summary

Correct the maximum permissible value for two Attributes of PlainTextExtension Element in the XML schema for GIF Image Metadata as recommended by the image specification.

Problem

. The maximum permissible value for two Attributes namely- Character Cell Width and Character Cell Height, of PlainTextExtension Element in the XML schema for GIF Image Metadata is incorrect.
. GIF Image specification version 89a mentions that the concerned attributes occupy 1 Byte each in the Plain Text Extension Block and hence, the maximum permissible value for these two attributes would be 255.

Solution

. Correct the maximum permissible value for the concerned Attributes in the XML schema for GIF Image Metadata.
. The XML schema for GIF Image metadata is documented in a HTML file that can be located at- src/java.desktop/share/classes/javax/imageio/metadata/doc-files/gif_metadata.html
. The proposed solution and relevant code changes have been reviewed for correctness.
. Review discussions can be found at- http://mail.openjdk.java.net/pipermail/2d-dev/2018-March/009030.html

Specification

File: src/java.desktop/share/classes/javax/imageio/metadata/doc-files/gif_metadata.html

     289       <!ATTLIST "PlainTextExtension" "characterCellWidth" #CDATA
     290          #REQUIRED>
     291         <!-- The width of a character cell -->
     292         <!-- Data type: Integer -->
     293         <!-- Min value: 1 (inclusive) -->
--   294         <!-- Max value: 65535 (inclusive) -->
++   294         <!-- Max value: 255 (inclusive) -->

     295       <!ATTLIST "PlainTextExtension" "characterCellHeight" #CDATA
     296          #REQUIRED>
     297         <!-- The height of a character cell -->
     298         <!-- Data type: Integer -->
     299         <!-- Min value: 1 (inclusive) -->
--   300         <!-- Max value: 65535 (inclusive) -->
++   300         <!-- Max value: 255 (inclusive) -->