[OpenJDK 2D-Dev] [PATCH] 8236996: Incorrect Roboto font rendering on Windows with subpixel antialiasing

Tue Jan 14 16:14:53 UTC 2020

> So this is a workaround for a buggy font that doesn't play well with GDI ?

This is a workaround for all cases (or the vast majority of them) of broken
rendering reported by our customers. The case with Roboto is just the one we
have steps to reproduce for. There can be other cases where GDI's logic is
not
matched by JDK. Even if all of them are caused by 'mis-constructed' fonts,
I'm
afraid, this will not be considered as a good excuse by our customers, as
only
Java applications have such problems with these fonts.

See JDK-8192972, still unsolved in OpenJDK, as an example of the problems
which
will be, at least partially, solved with this fix (correct glyphs will be
rendered, albeit using FreeType).

I did test the fix with fonts preinstalled in Windows 10. Fallback was
actually
triggered for one font (bold italic 'Segoe UI Semibold'), which is not a
'false'
positive, but actually a manifestation of another JDK bug from the same
family -
I've just raised https://bugs.openjdk.java.net/browse/JDK-8237085 for it.
That's
yet another example of an issue which will be (mostly) solved by the
proposed
fix.

Using file length as a 'checksum' certainly doesn't guarantee we choose the
right font, but the probability of error is very low, and this value seems
to be
the best candidate in our circumstances in terms of cost vs. benefit. Even
if
the validation mistreats a different font (having the same length) as a
correct
one, we'll not be in a worse position than before.

Of course, there's a certain risk that rendering for unaffected fonts might
change, but, given quite straightforward contract of GetFontData function, I
would consider it very low.

> Since you aren't retrieving the data, just asking what the size is, I'd
expect
> it to be unmeasurable.

Well, we don't know how GetFontData works exactly, but it does seem to add
some
overhead. On my Windows 10 machine OpenJDK with the proposed fix yields
about 7%
larger result for the following benchmark program. The reported value does
fluctuate from run to run, but the impact of the fix seems to be larger
than the
fluctuations.

--- Benchmark source code ----
import java.awt.*;
import java.awt.font.GlyphVector;
import java.awt.image.BufferedImage;

public class PerfTestOneFont {
    private static final Font FONT = new Font("Segoe UI", Font.PLAIN, 12);

    public static void main(String[] args) {
        FONT.getFamily(); // preload font

        BufferedImage image = new BufferedImage(1, 1,
BufferedImage.TYPE_INT_RGB);
        Graphics2D g = image.createGraphics();
        g.setRenderingHint(RenderingHints.KEY_TEXT_ANTIALIASING,
                  RenderingHints.VALUE_TEXT_ANTIALIAS_LCD_HRGB);
        int glyphCount = FONT.getNumGlyphs();
        long startTime = System.currentTimeMillis();
        for (int glyphCode = 0; glyphCode < glyphCount; glyphCode++) {
            GlyphVector gv =
FONT.createGlyphVector(g.getFontRenderContext(),
                                       new int[]{glyphCode});
            g.drawGlyphVector(gv, 0, 0);
        }
        long endTime = System.currentTimeMillis();
        g.dispose();
        System.out.println(endTime - startTime);
    }
}
------------------------------

Best regards,
Dmitry Batrak

On Mon, Jan 13, 2020 at 11:09 PM Phil Race <philip.race at oracle.com> wrote:

> So this is a workaround for a buggy font that doesn't play well with GDI ?
>
> It does rely on the fonts always being different sizes which is highly
> likely if not guaranteed.
> I suppose it is OK so long as we aren't getting any "false" positives.
>
> What I mean is that almost no one will have these Roboto fonts
> installed, so the fix
> is solving a problem they don't have, but if it is wrong in some way,
> then they could lose
> GDI rendering of LCD glyphs and that could affect a lot of people.
>
> So have you tested this with the full set of Windows 10 fonts -
> including Indic, CJK, etc  - to be sure
> there are no cases where it fails for these or other spurious failures.
>
>  > As for performance impact, during testing I didn't observe average
> glyph generation time increase of more than 15%.
>
> Since you aren't retrieving the data, just asking what the size is, I'd
> expect it to be unmeasurable.
>
> -phil.
>
> On 1/13/20 1:25 AM, Dmitry Batrak wrote:
> > Hello,
> >
> > I'd like to submit a patch for JDK-8236996. I'm not a Committer, so
> > I'll need someone to sponsor this change.
> >
> > Issue: https://bugs.openjdk.java.net/browse/JDK-8236996
> > Webrev: http://cr.openjdk.java.net/~dbatrak/8236996/webrev.00/
> >
> > The problem described in JDK-8236996 is from a group of issues (see
> > also e.g. JDK-8078382 and JDK-8192972), where JDK
> > uses one font to perform char-to-glyph conversion, but GDI, when asked
> > to render the glyph is picking a different font,
> > leading to completely random glyphs being rendered, as char-to-glyph
> > mapping obviously differs for different fonts.
> >
> > Specific version of Roboto font, mentioned in JDK-8236996, is most
> > probably causing the issue because it's not following
> > the naming guidelines from OpenType specification
> > (https://docs.microsoft.com/en-us/typography/opentype/spec/name),
> > having more than 4 variants (regular, bold, italic and bold italic)
> > with the same 'Font Family name' (name ID = 1). So,
> > GDI gets confused and picks Roboto Black for rendering, when asked to
> > choose a regular font from Roboto family (Roboto
> > Black having weight of 400, just like Roboto Regular, probably adds to
> > the confusion).
> >
> > But the reasoning, given above, about the issue cause is only a guess.
> > GDI is not an open-source subsystem, so we cannot
> > know for sure how it selects the font for rendering, and cannot
> > implement matching logic in JDK. Ideally, we'd want to
> > select the font by specifying its file path, but that's not possible
> > with GDI. Luckily, it allows us to query file data
> > for the selected font using GetFontData function
> > (
> https://docs.microsoft.com/en-us/windows/win32/api/wingdi/nf-wingdi-getfontdata),
>
> > which we can use to validate that the
> > selected font is the one we need.
> >
> > The proposed solution is to check the file size of the font, selected
> > by GDI, before using it for rendering. If a mismatch
> > is detected, fallback to FreeType is performed. It can produce a
> > somewhat different glyph representation, but, at least,
> > the correct glyph will be rendered. For members of font collections,
> > file size for validation is calculated in a special
> > way, in accordance with GetFontData logic described in the
> > documentation. I've verified that it works for font collections
> > bundled with Windows 10.
> >
> > As for performance impact, during testing I didn't observe average
> > glyph generation time increase of more than 15%.
> > Taking glyph caching into account, it shouldn't be that significant
> > for typical UI applications, I think. Performance
> > impact can be made even smaller - by performing the validation only
> > once per font, but, I believe, having a Java
> > application always render correct glyphs (even if fonts are added or
> > removed while application is running) is more
> > important.
> >
> > Proposed patch doesn't add any tests, as reproducing the issue
> > requires installation of fonts. Existing automated
> > OpenJDK tests pass after the fix. Proposed approach has been used in
> > JetBrains Runtime without known issues for about 3
> > months in testing and for about 1 month in production.
> >
> > Best regards,
> > Dmitry Batrak
>
>
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