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refine dual source blending / subpixel aa and RGBA color interaction

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Previously, we'd unconditionally enable dual source blending for the
text foreground layer when rendering. That meant that if the user had
configured the fg color to include an alpha value it would get "stamped
through" the draw all the way to the background, making that whole pixel
take on that alpha value rather than allowing it to blend through the
way you might expect.

In prior releases that didn't matter, but since we now allow configuring
the fg color with alpha, and allow using escape sequences to set the fg
for a span to something with alpha, there is now a much higher chance of
something looking weird.

Dual source blending is only really needed for subpixel-aa and that
isn't enabled by default.

This commit changes the behavior to use regular alpha blending if the
main config (rather than a per-font override) hasn't set the freetype
load/render target to one that enables subpixel-aa.

That means that alpha channel values work as expected for fg color
by default.

If you want to enable subpixel-aa you need to enable it globally
and be aware that it will cause weirdness when trying to use alpha
channels for the fg text color.

The docs now also indicate this behavior.

This limitation could be removed by making text rendering significantly
more complex and I don't fancy doing that at this time.
This commit is contained in:
Wez Furlong 2022-08-06 17:41:44 -07:00
parent 4c1fd693b4
commit 28d803e3f4
2 changed files with 18 additions and 3 deletions
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6
docs/config/lua/config/freetype_load_target.md
View File

@ -18,4 +18,10 @@ See also [freetype_render_target](freetype_render_target.md) and
[freetype_load_flags](freetype_load_flags.md) for more advanced flags that can
be primarily used to influence font hinting.
Note: when using subpixel-rendering, it comes at the cost of the ability to
explicitly set the alpha channel for the text foreground color. You will need
to choose between using the alpha channel or using subpixel-rendering, and you
must select subpixel-rendering in your main configuration in order for the
correct render mode to activate: setting it only in a
[wezterm.font](../wezterm/font.md) override is not sufficient.

15
wezterm-gui/src/termwindow/render.rs
View File

@ -20,8 +20,8 @@ use ::window::glium::{uniform, BlendingFunction, LinearBlendingFactor, Surface};
use ::window::{glium, DeadKeyStatus, PointF, RectF, SizeF, WindowOps};
use anyhow::anyhow;
use config::{
ConfigHandle, Dimension, DimensionContext, HsbTransform, TabBarColors, TextStyle,
VisualBellTarget,
ConfigHandle, Dimension, DimensionContext, FreeTypeLoadTarget, HsbTransform, TabBarColors,
TextStyle, VisualBellTarget,
};
use euclid::num::Zero;
use mux::pane::Pane;
@ -1440,6 +1440,15 @@ impl super::TermWindow {
)
.to_arrays_transposed();
let use_subpixel = match self
.config
.freetype_render_target
.unwrap_or(self.config.freetype_load_target)
{
FreeTypeLoadTarget::HorizontalLcd | FreeTypeLoadTarget::VerticalLcd => true,
_ => false,
};
let dual_source_blending = glium::DrawParameters {
blend: glium::Blend {
color: BlendingFunction::Addition {
@ -1497,7 +1506,7 @@ impl super::TermWindow {
let (vertex_count, index_count) = vb.vertex_index_count();
if vertex_count > 0 {
let vertices = vb.current_vb();
let subpixel_aa = idx == 1;
let subpixel_aa = use_subpixel && idx == 1;
frame.draw(
vertices.slice(0..vertex_count).unwrap(),