coretext: PragmataPro with font-thicken=true looks wrong

[mitchellh]

Left is Ghostty, right is iTerm2

Maybe related:

• https://gitlab.com/gnachman/iterm2/-/issues/7032

It looks like the are two issues:

1. Our glyphs are being cut off at the top (see the o which makes it really obvious I think)
2. The thickness is blurry

[mitchellh]

The freetype font backend looks fine. It doesn't support font-thicken anyways but I'm not noticing any rendering issues. This helps narrow it down to CoreText.

[mitchellh]

Okay. Things I can confirm so far:

• With font-thicken=false, Ghostty, iTerm2, TextEdit have identical rendering down to the individual pixel. I inspected each pixel of a letter and the colors fully match and pixel dimensions/locations fully match.
• With font-thicken=true, things get messier and Ghostty looks different. Colors differ slightly and pixel locations differ slightly.

[mitchellh]

#2123 fixes the clipping issue.

Now our pixel locations match perfectly, but our rendering is slightly darker. I'm not sure why yet.

[mitchellh]

Okay, I've drawn directly to a CoreGraphics context and dumped the pixels and compared to what I'm pulling with DigitalColorMeter and the colors match exactly. So the question I have is: why is CoreGraphics giving us darker pixels than are showing up in TextEdit and iTerm2?

[mitchellh]

It's darker because of gamma correction (or rather, a lack of it). Even though we set the colorspace of our drawable surfaces, we are not performing gamma correction yet on the RGB color values set manually by users or TUI programs (i.e. background, foreground, the full color palette, etc.). I jammed in some jank gamma correction multipliers into the OpenGL shader and was able to get very close values to macOS. I'm pretty sure this is the issue.

[mitchellh]

Dedicated issue for gamma correction in #2125.

The remainder of this issue I believer is resolved. Our pixels are now in roughly identical positions between iTerm and TextEdit, our colors are just very slightly off (we skew darker which makes things look thicker).

Note I say "roughly identical": the dimensions are identical, but there are cases where we have a pixel here or a pixel there difference. When inspecting under something like DigitalColorMeter, I see the same thing with iTerm and TextEdit depending on font sizes too. Ultimately, I don't think these single pixel differences will make a noticeable difference, the major difference now is color.

[madig]

The freetype font backend looks fine. It doesn't support font-thicken anyways

It should, see https://freetype.org/freetype2/docs/reference/ft2-properties.html#no-stem-darkening. Granted, the quality is uneven across format drivers, but it exists :)