[cairo] FT_Bitmap_Convert, and FreeType's bitmap format vs Cairo's

Hin-Tak Leung htl10 at users.sourceforge.net
Tue Apr 25 02:35:59 UTC 2017


FT_Bitmap_Convert's documentation is somewhat lacking in description of what it would do to

The behavior for FT_PIXEL_MODE_LCD and FT_PIXEL_MODE_LCD_V seems to be
a bit unexpected. It copies the pixel buffer as is, but changes the pixel mode from

afaic, FT_PIXEL_MODE_BGRA is flattened and converted to grays, which is sort of reasonable behavior.

I was somewhat expecting these three to throw Invalid Arg error, actually.

The reason why I was looking at FT_Bitmap_Convert is that I was looking for converting
FreeType's bitmap format vs Cairo's . I found only the 8-bit gray format works. The 1-bit format
has the wrong bit order on little-endian platforms - freetype's is always MSB first, while cairo's is host-order
( https://github.com/ldo/python_freetype/issues/1
https://github.com/ldo/python_freetype_examples/issues/1 )
Cairo does not have a 24-bit format - its RGB24 format is 32-bit with the highest 8-bit not used.

I have worked around the 1-bit problem with libtiff's TIFFReverseBits (any bitswapping routine from a common
library would do - I was trying to stay within python/freetype-py/pycairo without writing new C code)
- and I was hoping to overload FT_Bitmap_Convert()
if it currently rejects FT_PIXEL_MODE_BGRA to convert to cairo's ARGB32 , but that's dead-end.

Anyway, anybody has any bright idea about converting between freetype's the 3 24-bit/32-bit bitmap buffers
to cairo's RGB24 or ARGB32, without writing custom C code? And pwehaps also solving the 1-bit issue better?

I am hoping to add a

FT_Bitmap_Convert_More(...,  FT_PIXEL_MODE)

and perhaps adds a few more FT_PIXEL_MODE_* enums?

Also I am wondering about a little drawing issue I have - I want to scale a freetype bitmap up and draw a larger version of it (useful for illustrations for rasterization issues...) but it seems that cairo always anti-alias the edges at the original resolution, and I haven't figured out how to turn it off yet.
Let's say, I have rendered a glyph to a 24x24 grid in FreeType, and want to draw that in cairo to 480 x 480 . I'd like cairo to render at the destination resolution - i.e. do whatever at the 1-pixel boundary in 480x480, but it seems that it is doing it in the original grid, which blows up to be extra blurry at ~20 pixels wide in 480x480. Here is how it looks (the corner S is original resolution drawn as is):


This is from a word-in-progress rewrite of freetype-py's glyph-alpha/glyph-monochrome examples which uses numpy and matplotlib, to using pycairo for drawing.


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