[OpenJDK 2D-Dev] RFR: 8263583: Emoji rendering on macOS

[Alexey Ushakov]

On Mon, 15 Mar 2021 10:25:50 GMT, Dmitry Batrak <dbatrak at openjdk.org> wrote:

> This is the implementation used by JetBrains Runtime for the last 4 years, after some cleanup, and with one problem,
> found while preparing the pull request, fixed.
> Even though typical scenarios for a UI application should be covered, it's not a complete solution. In particular, emoji-s
> still won't be rendered for large font sizes (more than 100pt), and for non-trivial composite/painting modes.
> Notable implementation details are listed below.
> 
> **Glyph image generation**
> 
> Deprecated CGContextShowGlyphsAtPoint function, used by JDK on macOS to render text, cannot render emojis,
> CTFontDrawGlyphs is used instead. It ignores the scale component of text transformation matrix, so a 'real-sized'
> CTFont object should be passed to it. The same sizing procedure is done when calculating glyph metrics, because they
> are not scaled proportionally with font size (as they do for vector fonts).
> 
> **Glyph image storage**
> 
> Existing GlyphInfo structure is used to store color glyph image. Color glyph can be distinguished by having 4 bytes
> of storage per pixel. Color components are stored in pre-multiplied alpha format.
> 
> **Glyph rendering**
> 
> Previously, GlyphList instance always contained glyphs in the same format (solid, grayscale or LCD), determined by the
> effective rendering hint. Now the renderers must be prepared to GlyphList having 'normal' glyphs interspersed with
> color glyphs (they can appear due to font fallback). This isn't a problem for OpenGL renderer (used for on-screen painting),
> but GlyphListLoopPipe-based renderers (used for off-screen painting) needed an adjustment to be able to operate on
> specific segments of GlyphList.
> As an incidental optimization, calculation of GlyphList bounds ('getBounds' method) is performed now only when needed
> (most text renderers don't need this information).
> Speaking of the actual rendering of the glyph image, it's done by the straightforward glDrawPixels call in OpenGL renderer,
> and by re-using existing Blit primitive in off-screen renderers.
> 
> **Testing**
> 
> There's no good way to test the new functionality automatically, but I've added a test verifying that 'something' is
> rendered for the emoji character, when painting to BufferedImage.
> 
> Existing tests pass after the change.

Looks good

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PR: https://git.openjdk.java.net/jdk/pull/3007