JBIG2 symbol offsets

[janpe2]

JBIG2 text regions have a symbol positioning problem:

The top pixel row of symbols is missing. The row goes outside the text region because symbols are positioned one pixel too high. Here is another one:

jbig2_symbol_offset.pdf

In JBIG2 the REFCORNER value specifies which of the four corners of a symbol bitmap is positioned at the desired point (x, y). In practice (and in PDF.js) bitmaps are drawn starting from the left-top corner. If REFCORNER is other than left-top, some offsets are needed. Currently the JBIG2 decoder uses offsets that are one pixel too large. Thus the issue appears. Most often REFCORNER is left-bottom, like in my PDFs above, so only the vertical position is wrong.

When symbols are drawn, the correct offsets are one less than the symbol width and height. For example, the vertical offset when REFCORNER is left-bottom:

Typically JBIG2 images have a high resolution, so one pixel errors in symbol positioning can't be easily seen. (And symbols are rarely so near the edge of the text region that they would be clipped.)

This PR may cause JBIG2-related regression tests to fail because symbols will be moved one pixel. The new rendering is the correct one.

[Snuffleupagus]

/botio-linux preview

[pdfjsbot]

From: Bot.io (Linux m4)

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Received

Command cmd_preview from @Snuffleupagus received. Current queue size: 0

Live output at: http://54.67.70.0:8877/7b03522e1835136/output.txt

[pdfjsbot]

From: Bot.io (Linux m4)

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Success

Full output at http://54.67.70.0:8877/7b03522e1835136/output.txt

Total script time: 2.31 mins

Published

• Viewer: http://54.67.70.0:8877/7b03522e1835136/web/viewer.html
• Extension: http://54.67.70.0:8877/7b03522e1835136/extensions/firefox/pdf.js.xpi

[Snuffleupagus]

Conflicting files
test/pdfs/.gitignore
test/test_manifest.json

@janpe2 Could you please resolve these merge conflicts, so that we can test this patch? :-)
A small piece of advice (based on experience): To avoid having to rebase patches a lot because of merge conflicts in those two files in particular, try adding new test entries in more "random" positions rather than at the end of those files.

[Snuffleupagus]

/botio test

[pdfjsbot]

From: Bot.io (Linux m4)

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Received

Command cmd_test from @Snuffleupagus received. Current queue size: 0

Live output at: http://54.67.70.0:8877/6c6931fd5ad846d/output.txt

[pdfjsbot]

From: Bot.io (Windows)

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Received

Command cmd_test from @Snuffleupagus received. Current queue size: 1

Live output at: http://54.215.176.217:8877/89fecdff5743b9b/output.txt

[pdfjsbot]

From: Bot.io (Linux m4)

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Failed

Full output at http://54.67.70.0:8877/6c6931fd5ad846d/output.txt

Total script time: 16.78 mins

• Font tests: Passed
• Unit tests: Passed
• Regression tests: FAILED

Image differences available at: http://54.67.70.0:8877/6c6931fd5ad846d/reftest-analyzer.html#web=eq.log

[pdfjsbot]

From: Bot.io (Windows)

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Failed

Full output at http://54.215.176.217:8877/89fecdff5743b9b/output.txt

Total script time: 29.46 mins

• Font tests: Passed
• Unit tests: Passed
• Regression tests: FAILED

Image differences available at: http://54.215.176.217:8877/89fecdff5743b9b/reftest-analyzer.html#web=eq.log

[timvandermeij]

/botio makeref

[pdfjsbot]

From: Bot.io (Windows)

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Received

Command cmd_makeref from @timvandermeij received. Current queue size: 1

Live output at: http://54.215.176.217:8877/a1f650355ddcd56/output.txt

[pdfjsbot]

From: Bot.io (Linux m4)

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Received

Command cmd_makeref from @timvandermeij received. Current queue size: 0

Live output at: http://54.67.70.0:8877/d320c13b12ebc42/output.txt

[pdfjsbot]

From: Bot.io (Linux m4)

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Success

Full output at http://54.67.70.0:8877/d320c13b12ebc42/output.txt

Total script time: 15.79 mins

• Lint: Passed
• Make references: Passed
• Check references: Passed

[pdfjsbot]

From: Bot.io (Windows)

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Success

Full output at http://54.215.176.217:8877/a1f650355ddcd56/output.txt

Total script time: 27.69 mins

• Lint: Passed
• Make references: Passed
• Check references: Passed

[timvandermeij]

Thank you for fixing this!