Is Glyph.y described in Y up or down coordinate system?

[maitbayev]

It’s not clear from the documentation which coordinate system Glyph.y uses.

On one hand, it appears to be defined in the glyph coordinate system, where the origin is at the bottom-left and Y points upwards. Several references seem to support this:

1. parley/examples/swash_render/src/main.rs

   Line 254 in e581c08

   let glyph_y = run_y - glyph.y + (padding as f32);

2. parley/examples/tiny_skia_render/src/main.rs

   Line 175 in e581c08

   let glyph_y = run_y - glyph.y + padding as f32;

3. More importantly,

   parley/parley/src/layout/data.rs

   Line 754 in e581c08

   y: (glyph_pos.y_offset as f32) * scale_factor,

   , which indicates that Parley follows the coordinate conventions defined by HarfRust.

On the other hand, some parts of the code suggest that it might instead use a screen coordinate system (origin at the top-left, Y downwards):

1. parley/parley/src/layout/line.rs

   Line 231 in e581c08

   g.y += baseline;

My local experiments also seem to suggest that the Glyph.y is defined in the former coordinate system. Then, is the latter a bug?

[nicoburns]

I believe it's upwards from the baseline (which is also why the baseline does get added - it's the origin)

[maitbayev]

@nicoburns if it is upwards, then it should be subtracted from the baseline, since baseline defined in screen coordinate system.

[nicoburns]

@nicoburns if it is upwards, then it should be subtracted from the baseline, since baseline defined in screen coordinate system.

The examples you include above are all subtracting glyph.y, no?

[maitbayev]

@nicoburns agreed, luckily almost all examples that I included looks correct, except the last one in positioned_glyphs:

1. parley/parley/src/layout/line.rs

   Line 231 in e581c08

   g.y += baseline;

This one looks wrong to me, hence this confusion and github issue.