broken ColorTest with cpu rendering

[yjh0502]

While tracking down rendering errors, I found that color rendering is broken with cpu rendering (cpu_fix).
Here's a code snippet to render ColorTest.

use egui_skia::rasterize;
use skia_safe::{EncodedImageFormat, Paint, Point};
use std::fs::File;
use std::io::Write;

pub fn main() {
    let mut demo = egui_demo_lib::ColorTest::default();

    let mut surface = rasterize(
        (800, 2000),
        |ctx| {
            egui::CentralPanel::default().show(ctx, |ui| {
                demo.ui(ui);
            });
        },
        None,
    );

    let data = surface
        .image_snapshot()
        .encode_to_data(EncodedImageFormat::PNG)
        .expect("Failed to encode image");

    File::create("output.png")
        .unwrap()
        .write_all(&data)
        .unwrap();

    println!("wrote output.png");
}

This is cpu-rendered image, generate from a code above, which shows errors.

Here's a side-by-side screenshot, comparing cpu-rendered with webgl one (from https://egui.rs)

[yjh0502]

Some errors could be fixed by un-premultiplying colors from egui.

diff --git a/src/painter.rs b/src/painter.rs
index 5c07f5b..8696332 100644
--- a/src/painter.rs
+++ b/src/painter.rs
@@ -205,11 +205,19 @@ impl Painter {
 
                             pos.push(Point::new(fixed_pos.x, fixed_pos.y));
                             texs.push(Point::new(v.uv.x, v.uv.y));
+
+                            let c = v.color;
+                            let c = Color::from_argb(c.a(), c.r(), c.g(), c.b());
+                            // un-premultply color
+                            let mut cf = skia_safe::Color4f::from(c);
+                            cf.r /= cf.a;
+                            cf.g /= cf.a;
+                            cf.b /= cf.a;
                             colors.push(Color::from_argb(
-                                v.color.a(),
-                                v.color.r(),
-                                v.color.g(),
-                                v.color.b(),
+                                c.a(),
+                                (cf.r * 255.0) as u8,
+                                (cf.g * 255.0) as u8,
+                                (cf.b * 255.0) as u8,
                             ));
                         });

[lucasmerlin]

Hi! I applied your fix in #7, improving on the color test results. But as you already mentioned it's not perfect yet so I'll leave this issue open for now.
I also tested if I could get this to work by changing the type of the created skia texture but it didn't seem to make any difference.
Happy to accept a PR if someone figures out how to fix the additive blending and linear interpolation test cases!

[jb55]

The colors on rendered images are all messed up (weird pink tinge), I wonder if this is related?

[jb55]

I tried rendering a gradient texture and when I sample the color with a color picker, the red and the blue components appear to be swapped. I fixed it with this:

let scol = [0x1E, 0x55, 0xFF];
let ecol = [0xFA, 0x0D, 0xD4];

 // TODO: skia has r/b colors swapped for some reason, fix this
let start_color = Color32::from_rgb(scol[2], scol[1], scol[0]);
let end_color = Color32::from_rgb(ecol[2], ecol[1], ecol[0]);

let gradient = Gradient::linear(start_color, end_color);

before: https://cdn.jb55.com/s/0cdefad30a081797.png
after swapping r/b: https://cdn.jb55.com/s/2da42af5f9f69756.png