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Drop the FT2Font intermediate buffer. #30059

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Drop the FT2Font intermediate buffer. #30059

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@anntzer anntzer commented May 16, 2025 •
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Directly render FT glyphs to the Agg buffer. In particular, this naturally provides, with no extra work, subpixel positioning of glyphs (closing #29551) (this could also have been implemented in the old framework, but would have required careful tracking of subpixel offets).

Note that rendering of mathtext boxes (e.g., fraction bars) is currently missing, but should be "easy" (the main question being whether/how to ensure proper hinting/alignment to pixel edges in the horizontal or vertical output cases). Likewise, all baseline images should be regenerated. Finally, the new APIs added to FT2Font are likely up to bikeshedding (but they are all private).

In practice this should get included (if we agree to go this way) into the FreeType update (#29816). This would also supersede the patch advertised at #29816 (comment).

test modified from #29551 to also include mathtext:

import matplotlib.pyplot as plt
import matplotlib.animation as manim

fig = plt.figure(figsize=(2, 2))
ts = [
    fig.text(0, 0, '$ABC123$'),
    fig.text(0, 0.2, 'ABC123'),
]
change = 0.001

def update(*args, **kwargs):
    global change
    for t in ts:
        x, y = t.get_position()
        if not (0 <= x <= 1 and 0 <= y <= 1):
            change *= -1
        t.set_x(x + change)
        t.set_y(y + change)
    return ts

ani = manim.FuncAnimation(fig, update, interval=20, frames=int(2/change),
                          cache_frame_data=False)
ani.save('text.gif')

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story645 commented May 16, 2025 •
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Would you be open to using a tracking issue or project board for the font work (sample categories: draft, waiting on other PRs, ready for review, please review first)?

I think breaking everything up into small PRs is fantastic but I'm finding it a bit overwhelming to keep track of the order for reviewing things (and figure I'm not alone here) and how these PRs relate to each other.

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anntzer commented May 16, 2025 •
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Sure, I'll do that. I'm sorry for the mess, but this is basically me trying to remember all the patches or even just ideas I have accumulated over the years that went nowhere because they would thrash all the baseline images, and that I now try to present given that I see an opportunity with the FreeType upgrade. Also, some of them are not fully complete patches but things that I believe can be made complete with relatively little effort (for some value of "little effort"...) simply because I do not have the time now to finalize them but want to put them up for discussion as the underlying idea should be clear.

Edit: see the wiki; attn @QuLogic as well too.

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story645 commented May 16, 2025 •
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I'm sorry for the mess, but

No apologies needed, I commend you for taking advantage of the freetype upgrade. And thanks for the wiki, though some reason can't link cleanly ☹️

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anntzer commented May 16, 2025

link fixed

@QuLogic QuLogic mentioned this pull request May 17, 2025
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@anntzer anntzer force-pushed the ft-direct-render branch 2 times, most recently from 8ad0be4 to d44360f Compare May 17, 2025 09:54
@anntzer
Drop the FT2Font intermediate buffer.
66fdae0 
Directly render FT glyphs to the Agg buffer.  In particular, this
naturally provides, with no extra work, subpixel positioning of glyphs
(which could also have been implemented in the old framework, but would
have required careful tracking of subpixel offets).

Note that all baseline images should be regenerated.  The new APIs added
to FT2Font are also up to bikeshedding (but they are all private).
@anntzer anntzer force-pushed the ft-direct-render branch from d44360f to 66fdae0 Compare May 17, 2025 09:55
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anntzer commented May 17, 2025

Edit: fraction bar rendering has been added too.

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QuLogic commented May 22, 2025

A few warnings from compiling locally:

[2/4] Compiling C++ object src/ft2font.cpython-313-x86_64-linux-gnu.so.p/ft2font_wrapper.cpp.o
../../src/ft2font_wrapper.cpp: In lambda function:
../../src/ft2font_wrapper.cpp:1785:22: warning: unused variable ‘error’ [-Wunused-variable]
 1785 |             if (auto error = FT_Load_Glyph(face, idx, static_cast<FT_Int32>(flags))) {
      |                      ^~~~~
../../src/ft2font_wrapper.cpp:1788:22: warning: unused variable ‘error’ [-Wunused-variable]
 1788 |             if (auto error = FT_Render_Glyph(face->glyph, FT_RENDER_MODE_NORMAL)) {
      |                      ^~~~~
../../src/ft2font_wrapper.cpp: In lambda function:
../../src/ft2font_wrapper.cpp:1804:26: warning: unused variable ‘error’ [-Wunused-variable]
 1804 |                 if (auto error = FT_Glyph_To_Bitmap(&g, FT_RENDER_MODE_NORMAL, &origin, 1)) {
      |                          ^~~~~

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anntzer commented May 22, 2025

This is semi-intentional: if these checks fail (error > 0) then the error should really be raised using throw_ft_error, not with a plain raise as I currently do, but throw_ft_error is not visible from ft2font_wrapper.cpp so some code needs to be moved around, e.g. make throw_ft_error an inline function defined in ft2font.h, or move the new implementations into real C++ FT2Font methods in ft2font.cpp and add wrapper lambdas in ft2font_wrapper.cpp, or (would be my preferred choice) switch error checking to the FT_CHECK macro used in mplcairo (see mplcairo's macros.h).
I left the warning as is just to keep this as a side discussion point, but it can trivially be suppressed by not capturing the error value too.

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@anntzer @story645 @QuLogic