ClipBoxFix

ClipBoxFix

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This parameter is meant to fix the problem that some programs mistakenly clipped bottom or top part of the text. Here is the principle:

Windows ClearType can generate a clipbox for every letter which indicates the exact rect of that letter. Some programs (like java, chrome which have their graphics render engines) need to cache the bitmap of letter ClearType generated, in order to accelerate their render speed. To save memory the cache consumes, they will create bitmaps with exactly the same size ClearType provided.

As you can see, MacType/FreeType often provides text result that is a bit larger than ClearType. This is generally because FreeType tend to use alpha pixels to make font shape as designed as possible while ClearType always do pixel snapping without alpha. Thus the letter MacType generated won't fit that clipbox ClearType generated, but fortunately, it is most likely only 1px larger.

To solve this problem, we enlarge the clipbox ClearType generated by 2px in vertical. This works fine in most cases, but some other programs use the clipbox to calculate display layout, which will finally lead to a slightly different layout due to the enlargement we do.

Conclusion:

Set clipboxfix to 1 (or left it unset) is fine in most cases.

Set to 0 if you encounter the problem.