Fix alpha blending in WebGL2 backend (#650)

Add a render-to-texture step with an sRGBA8 texture

egui_demo_lib/src/apps/color_test.rs

@@ -39,9 +39,8 @@ impl epi::App for ColorTest {
             if frame.is_web() {
                 ui.colored_label(
                     RED,
-                    "NOTE: The WebGL backend does NOT pass the color test."
+                    "NOTE: The WebGL1 backend does NOT pass the color test. The WebGL2 backend does."
                 );
-                ui.small("This is because WebGL does not support a linear framebuffer blending (not even WebGL2!).\nMaybe when WebGL3 becomes mainstream in 2030 the web can finally get colors right?");
                 ui.separator();
             }
             ScrollArea::auto_sized().show(ui, |ui| {

egui_demo_lib/src/apps/demo/painting.rs

@@ -12,7 +12,7 @@ impl Default for Painting {
     fn default() -> Self {
         Self {
             lines: Default::default(),
-            stroke: Stroke::new(2.0, Color32::LIGHT_BLUE), // Thin strokes looks bad on web
+            stroke: Stroke::new(1.0, Color32::LIGHT_BLUE),
         }
     }
 }

egui_web/CHANGELOG.md

@@ -5,6 +5,9 @@ All notable changes to the `egui_web` integration will be noted in this file.
 
 ## Unreleased
 
+### Fixed 🐛
+* Fix alpha blending for WebGL2 backend, now having identical results as egui_glium
+
 
 ## 0.14.0 - 2021-08-24
 

egui_web/Cargo.toml

@@ -92,11 +92,13 @@ features = [
   "TouchList",
   "WebGl2RenderingContext",
   "WebGlBuffer",
+  "WebGlFramebuffer",
   "WebGlProgram",
   "WebGlRenderingContext",
   "WebGlShader",
   "WebGlTexture",
   "WebGlUniformLocation",
+  "WebGlVertexArrayObject",
   "WheelEvent",
   "Window",
 ]

egui_web/src/shader/fragment_100es.glsl

@@ -30,7 +30,7 @@ vec4 linear_from_srgba(vec4 srgba) {
 }
 
 void main() {
-  // We must decode the colors, since WebGL doesn't come with sRGBA textures:
+  // We must decode the colors, since WebGL1 doesn't come with sRGBA textures:
   vec4 texture_rgba = linear_from_srgba(texture2D(u_sampler, v_tc) * 255.0);
 
   /// Multiply vertex color with texture color (in linear space).

egui_web/src/shader/fragment_300es.glsl

@@ -1,45 +0,0 @@
-precision mediump float;
-uniform sampler2D u_sampler;
-varying vec4 v_rgba;
-varying vec2 v_tc;
-
-// 0-255 sRGB  from  0-1 linear
-vec3 srgb_from_linear(vec3 rgb) {
-  bvec3 cutoff = lessThan(rgb, vec3(0.0031308));
-  vec3 lower = rgb * vec3(3294.6);
-  vec3 higher = vec3(269.025) * pow(rgb, vec3(1.0 / 2.4)) - vec3(14.025);
-  return mix(higher, lower, vec3(cutoff));
-}
-
-// 0-255 sRGBA  from  0-1 linear
-vec4 srgba_from_linear(vec4 rgba) {
-  return vec4(srgb_from_linear(rgba.rgb), 255.0 * rgba.a);
-}
-
-void main() {
-  // The texture is set up with `SRGB8_ALPHA8`, so no need to decode here!
-  vec4 texture_rgba = texture2D(u_sampler, v_tc);
-
-  /// Multiply vertex color with texture color (in linear space).
-  gl_FragColor = v_rgba * texture_rgba;
-
-  // WebGL doesn't support linear blending in the framebuffer,
-  // so we do a hack here where we change the premultiplied alpha
-  // to do the multiplication in gamma space instead:
-
-  // Unmultiply alpha:
-  if (gl_FragColor.a > 0.0) {
-    gl_FragColor.rgb /= gl_FragColor.a;
-  }
-
-  // Empiric tweak to make e.g. shadows look more like they should:
-  gl_FragColor.a *= sqrt(gl_FragColor.a);
-
-  // To gamma:
-  gl_FragColor = srgba_from_linear(gl_FragColor) / 255.0;
-
-  // Premultiply alpha, this time in gamma space:
-  if (gl_FragColor.a > 0.0) {
-    gl_FragColor.rgb *= gl_FragColor.a;
-  }
-}

egui_web/src/shader/main_fragment_300es.glsl

@@ -0,0 +1,13 @@
+precision mediump float;
+uniform sampler2D u_sampler;
+varying vec4 v_rgba;
+varying vec2 v_tc;
+
+void main() {
+  // The texture is set up with `SRGB8_ALPHA8`, so no need to decode here!
+  vec4 texture_rgba = texture2D(u_sampler, v_tc);
+
+  // Multiply vertex color with texture color (in linear space).
+  // Linear color is written and blended in Framebuffer and converted to sRGB later
+  gl_FragColor = v_rgba * texture_rgba;
+}

egui_web/src/shader/post_fragment_300es.glsl

@@ -0,0 +1,22 @@
+precision mediump float;
+uniform sampler2D u_sampler;
+varying vec2 v_tc;
+
+// 0-255 sRGB  from  0-1 linear
+vec3 srgb_from_linear(vec3 rgb) {
+    bvec3 cutoff = lessThan(rgb, vec3(0.0031308));
+    vec3 lower = rgb * vec3(3294.6);
+    vec3 higher = vec3(269.025) * pow(rgb, vec3(1.0 / 2.4)) - vec3(14.025);
+    return mix(higher, lower, vec3(cutoff));
+}
+
+// 0-255 sRGBA  from  0-1 linear
+vec4 srgba_from_linear(vec4 rgba) {
+    return vec4(srgb_from_linear(rgba.rgb), 255.0 * rgba.a);
+}
+
+void main() {
+    gl_FragColor = texture2D(u_sampler, v_tc);
+
+    gl_FragColor = srgba_from_linear(gl_FragColor) / 255.;
+}

egui_web/src/shader/post_vertex_300es.glsl

@@ -0,0 +1,8 @@
+precision mediump float;
+attribute vec2 a_pos;
+varying vec2 v_tc;
+
+void main() {
+    gl_Position = vec4(a_pos * 2. - 1., 0.0, 1.0);
+    v_tc = a_pos;
+}

egui_web/src/webgl2.rs

@@ -3,7 +3,10 @@
 use {
     js_sys::WebAssembly,
     wasm_bindgen::{prelude::*, JsCast},
-    web_sys::{WebGl2RenderingContext, WebGlBuffer, WebGlProgram, WebGlShader, WebGlTexture},
+    web_sys::{
+        WebGl2RenderingContext, WebGlBuffer, WebGlFramebuffer, WebGlProgram, WebGlShader,
+        WebGlTexture, WebGlVertexArrayObject,
+    },
 };
 
 use egui::{
@@ -22,6 +25,7 @@ pub struct WebGl2Painter {
     pos_buffer: WebGlBuffer,
     tc_buffer: WebGlBuffer,
     color_buffer: WebGlBuffer,
+    post_process: PostProcess,
 
     egui_texture: WebGlTexture,
     egui_texture_version: Option<u64>,
@@ -62,12 +66,12 @@ impl WebGl2Painter {
         let vert_shader = compile_shader(
             &gl,
             Gl::VERTEX_SHADER,
-            include_str!("shader/vertex_300es.glsl"),
+            include_str!("shader/main_vertex_300es.glsl"),
         )?;
         let frag_shader = compile_shader(
             &gl,
             Gl::FRAGMENT_SHADER,
-            include_str!("shader/fragment_300es.glsl"),
+            include_str!("shader/main_fragment_300es.glsl"),
         )?;
 
         let program = link_program(&gl, [vert_shader, frag_shader].iter())?;
@@ -76,6 +80,9 @@ impl WebGl2Painter {
         let tc_buffer = gl.create_buffer().ok_or("failed to create tc_buffer")?;
         let color_buffer = gl.create_buffer().ok_or("failed to create color_buffer")?;
 
+        let post_process =
+            PostProcess::new(gl.clone(), canvas.width() as i32, canvas.height() as i32)?;
+
         Ok(WebGl2Painter {
             canvas_id: canvas_id.to_owned(),
             canvas,
@@ -85,6 +92,7 @@ impl WebGl2Painter {
             pos_buffer,
             tc_buffer,
             color_buffer,
+            post_process,
             egui_texture,
             egui_texture_version: None,
             user_textures: Default::default(),
@@ -368,8 +376,7 @@ impl crate::Painter for WebGl2Painter {
         }
 
         let mut pixels: Vec<u8> = Vec::with_capacity(texture.pixels.len() * 4);
-        let font_gamma = 1.0 / 2.2; // HACK due to non-linear framebuffer blending.
+        for srgba in texture.srgba_pixels(1.0) {
-        for srgba in texture.srgba_pixels(font_gamma) {
             pixels.push(srgba.r());
             pixels.push(srgba.g());
             pixels.push(srgba.b());
@@ -429,6 +436,9 @@ impl crate::Painter for WebGl2Painter {
 
         let gl = &self.gl;
 
+        self.post_process
+            .begin(self.canvas.width() as i32, self.canvas.height() as i32)?;
+
         gl.enable(Gl::SCISSOR_TEST);
         gl.disable(Gl::CULL_FACE); // egui is not strict about winding order.
         gl.enable(Gl::BLEND);
@@ -485,10 +495,174 @@ impl crate::Painter for WebGl2Painter {
                 ));
             }
         }
+
+        self.post_process.end();
+
         Ok(())
     }
 }
 
+/// Uses a framebuffer to render everything in linear color space and convert it back to sRGB
+/// in a separate "post processing" step
+struct PostProcess {
+    gl: Gl,
+    pos_buffer: WebGlBuffer,
+    index_buffer: WebGlBuffer,
+    vao: WebGlVertexArrayObject,
+    texture: WebGlTexture,
+    texture_size: (i32, i32),
+    fbo: WebGlFramebuffer,
+    program: WebGlProgram,
+}
+
+impl PostProcess {
+    fn new(gl: Gl, width: i32, height: i32) -> Result<PostProcess, JsValue> {
+        let fbo = gl
+            .create_framebuffer()
+            .ok_or("failed to create framebuffer")?;
+        gl.bind_framebuffer(Gl::FRAMEBUFFER, Some(&fbo));
+
+        let texture = gl.create_texture().unwrap();
+        gl.bind_texture(Gl::TEXTURE_2D, Some(&texture));
+        gl.tex_parameteri(Gl::TEXTURE_2D, Gl::TEXTURE_WRAP_S, Gl::CLAMP_TO_EDGE as i32);
+        gl.tex_parameteri(Gl::TEXTURE_2D, Gl::TEXTURE_WRAP_T, Gl::CLAMP_TO_EDGE as i32);
+        gl.tex_parameteri(Gl::TEXTURE_2D, Gl::TEXTURE_MIN_FILTER, Gl::NEAREST as i32);
+        gl.tex_parameteri(Gl::TEXTURE_2D, Gl::TEXTURE_MAG_FILTER, Gl::NEAREST as i32);
+        gl.pixel_storei(Gl::UNPACK_ALIGNMENT, 1);
+        gl.tex_image_2d_with_i32_and_i32_and_i32_and_format_and_type_and_opt_u8_array(
+            Gl::TEXTURE_2D,
+            0,
+            Gl::SRGB8_ALPHA8 as i32,
+            width,
+            height,
+            0,
+            Gl::RGBA,
+            Gl::UNSIGNED_BYTE,
+            None,
+        )
+        .unwrap();
+        gl.framebuffer_texture_2d(
+            Gl::FRAMEBUFFER,
+            Gl::COLOR_ATTACHMENT0,
+            Gl::TEXTURE_2D,
+            Some(&texture),
+            0,
+        );
+
+        gl.bind_texture(Gl::TEXTURE_2D, None);
+        gl.bind_framebuffer(Gl::FRAMEBUFFER, None);
+
+        let vert_shader = compile_shader(
+            &gl,
+            Gl::VERTEX_SHADER,
+            include_str!("shader/post_vertex_300es.glsl"),
+        )?;
+        let frag_shader = compile_shader(
+            &gl,
+            Gl::FRAGMENT_SHADER,
+            include_str!("shader/post_fragment_300es.glsl"),
+        )?;
+        let program = link_program(&gl, [vert_shader, frag_shader].iter())?;
+
+        let vao = gl.create_vertex_array().ok_or("failed to create vao")?;
+        gl.bind_vertex_array(Some(&vao));
+
+        let positions = vec![0u8, 0, 1, 0, 0, 1, 1, 1];
+
+        let indices = vec![0u8, 1, 2, 1, 2, 3];
+
+        let pos_buffer = gl.create_buffer().ok_or("failed to create pos_buffer")?;
+        gl.bind_buffer(Gl::ARRAY_BUFFER, Some(&pos_buffer));
+        gl.buffer_data_with_u8_array(Gl::ARRAY_BUFFER, &positions, Gl::STATIC_DRAW);
+
+        let a_pos_loc = gl.get_attrib_location(&program, "a_pos");
+        assert!(a_pos_loc >= 0);
+        gl.vertex_attrib_pointer_with_i32(a_pos_loc as u32, 2, Gl::UNSIGNED_BYTE, false, 0, 0);
+        gl.enable_vertex_attrib_array(a_pos_loc as u32);
+
+        gl.bind_buffer(Gl::ARRAY_BUFFER, None);
+
+        let index_buffer = gl.create_buffer().ok_or("failed to create index_buffer")?;
+        gl.bind_buffer(Gl::ELEMENT_ARRAY_BUFFER, Some(&index_buffer));
+        gl.buffer_data_with_u8_array(Gl::ELEMENT_ARRAY_BUFFER, &indices, Gl::STATIC_DRAW);
+
+        gl.bind_vertex_array(None);
+        gl.bind_buffer(Gl::ELEMENT_ARRAY_BUFFER, None);
+
+        Ok(PostProcess {
+            gl,
+            pos_buffer,
+            index_buffer,
+            vao,
+            texture,
+            texture_size: (width, height),
+            fbo,
+            program,
+        })
+    }
+
+    fn begin(&mut self, width: i32, height: i32) -> Result<(), JsValue> {
+        let gl = &self.gl;
+
+        if (width, height) != self.texture_size {
+            gl.bind_texture(Gl::TEXTURE_2D, Some(&self.texture));
+            gl.pixel_storei(Gl::UNPACK_ALIGNMENT, 1);
+            gl.tex_image_2d_with_i32_and_i32_and_i32_and_format_and_type_and_opt_u8_array(
+                Gl::TEXTURE_2D,
+                0,
+                Gl::SRGB8_ALPHA8 as i32,
+                width,
+                height,
+                0,
+                Gl::RGBA,
+                Gl::UNSIGNED_BYTE,
+                None,
+            )?;
+            gl.bind_texture(Gl::TEXTURE_2D, None);
+
+            self.texture_size = (width, height);
+        }
+
+        gl.bind_framebuffer(Gl::FRAMEBUFFER, Some(&self.fbo));
+
+        Ok(())
+    }
+
+    fn end(&self) {
+        let gl = &self.gl;
+
+        gl.bind_framebuffer(Gl::FRAMEBUFFER, None);
+        gl.disable(Gl::SCISSOR_TEST);
+
+        gl.use_program(Some(&self.program));
+
+        gl.active_texture(Gl::TEXTURE0);
+        gl.bind_texture(Gl::TEXTURE_2D, Some(&self.texture));
+        let u_sampler_loc = gl.get_uniform_location(&self.program, "u_sampler").unwrap();
+        gl.uniform1i(Some(&u_sampler_loc), 0);
+
+        gl.bind_vertex_array(Some(&self.vao));
+
+        gl.draw_elements_with_i32(Gl::TRIANGLES, 6, Gl::UNSIGNED_BYTE, 0);
+
+        gl.bind_texture(Gl::TEXTURE_2D, None);
+        gl.bind_vertex_array(None);
+        gl.use_program(None);
+    }
+}
+
+impl Drop for PostProcess {
+    fn drop(&mut self) {
+        let gl = &self.gl;
+        gl.delete_vertex_array(Some(&self.vao));
+        gl.delete_buffer(Some(&self.pos_buffer));
+        gl.delete_buffer(Some(&self.index_buffer));
+        gl.delete_program(Some(&self.program));
+        gl.delete_framebuffer(Some(&self.fbo));
+        gl.delete_texture(Some(&self.texture));
+    }
+}
+
 fn compile_shader(
     gl: &WebGl2RenderingContext,
     shader_type: u32,