Getting Pixel Color

[elwerene]

I'm trying to read the color at certain points after drawing. Do you know how I could archive that?

I found frame.gl() but I always get the same color "r: 9, g: 9, b: 9" and I don't know how to continue/debug:

if let Some(pos) = ctx.pointer_latest_pos() {
    let (r, g, b) = unsafe {
        let pos = pos.round();
        let mut buf = [0u8; 3];
        let pixels = glow::PixelPackData::Slice(&mut buf[..]);
        frame.gl().read_pixels(
            pos.x as i32,
            pos.y as i32,
            1,
            1,
            glow::RGB,
            glow::UNSIGNED_BYTE,
            pixels,
        );
        (buf[0], buf[1], buf[2])
    };
    egui::Window::new("overlay")
        .fixed_pos(pos - Vec2::new(50.0, 40.0))
        .resizable(false)
        .title_bar(false)
        .collapsible(false)
        .show(ctx, |ui| {
            ui.label(format!("r: {r}, g: {g}, b: {b}"));
        });
}

[emilk]

eframe clears the color buffer before running App::update, so that's why you are seeing the same color: https://github.com/emilk/egui/blob/master/eframe/src/native/run.rs#L98-L109

We would need to add a App::post_rendering call or something for this

[elwerene]

@emilk I will create a merge request. But first It should run in my test. How would I calculate the gl coordinates?
I tried

let x = (pos.x * frame.info().native_pixels_per_point.unwrap_or(1.0)).round();
let y = ((frame.output.window_size.unwrap_or_default().y - pos.y) * frame.info().native_pixels_per_point.unwrap_or(1.0)).round();

Sadly the window size (for getting the height/inverting y) is never set?

[emilk]

If you add a post_rendering method, you can also add the necessary arguments? Perhaps PaintCallbackInfo can be reused. There is also ctx.input().screen_rect.size().

[elwerene]

See pull request: #1591

Working example with this pull request:

Self:

..
    pos: Option<Pos2>,
    color: String,
..

in update():

if let Some(pos) = ctx.pointer_latest_pos() {
            egui::Window::new("overlay")
                .fixed_pos(pos - Vec2::new(50.0, 40.0))
                .resizable(false)
                .title_bar(false)
                .collapsible(false)
                .show(ctx, |ui| {
                    ui.label(&self.color);
                });
        }

    fn post_rendering(&mut self, screen_size_px: [u32; 2], frame: &eframe::Frame) {
        if let Some(pos) = self.pos {
            let (r, g, b) = unsafe {
                let x = (pos.x * frame.info().native_pixels_per_point.unwrap_or(1.0)).round();
                let y = screen_size_px[1] as i32
                    - (pos.y * frame.info().native_pixels_per_point.unwrap_or(1.0)).round() as i32;
                dbg!(y);

                let mut buf = [0u8; 3];
                let pixels = glow::PixelPackData::Slice(&mut buf[..]);
                frame.gl().read_pixels(
                    x as i32,
                    y as i32,
                    1,
                    1,
                    glow::RGB,
                    glow::UNSIGNED_BYTE,
                    pixels,
                );
                (buf[0], buf[1], buf[2])
            };
            self.color = format!("r: {r}, g: {g}, b: {b}");
        }
    }