Use un-multiplied alpha in shader hooks

src/webgl/shaders/phong.frag

@@ -47,13 +47,13 @@ void main(void) {
   inputs.texCoord = vTexCoord;
   inputs.ambientLight = vAmbientColor;
   inputs.color = isTexture
-      // Textures come in with premultiplied alpha. To apply tint and still have
-      // premultiplied alpha output, we need to multiply the RGB channels by the
-      // tint RGB, and all channels by the tint alpha.
-      ? TEXTURE(uSampler, vTexCoord) * vec4(uTint.rgb/255., 1.) * (uTint.a/255.)
-      // Colors come in with unmultiplied alpha, so we need to multiply the RGB
-      // channels by alpha to convert it to premultiplied alpha.
-      : vec4(vColor.rgb * vColor.a, vColor.a);
+      ? TEXTURE(uSampler, vTexCoord) * uTint/255.
+      : vColor;
+  if (isTexture) {
+    // Textures come in with premultiplied alpha. Temporarily unpremultiply it
+    // so hooks users don't have to think about premultiplied alpha.
+    inputs.color.rgb /= inputs.color.a;
+  }
   inputs.shininess = uShininess;
   inputs.metalness = uMetallic;
   inputs.ambientMaterial = uHasSetAmbient ? uAmbientMatColor.rgb : inputs.color.rgb;
@@ -79,5 +79,6 @@ void main(void) {
   c.specular = specular;
   c.emissive = inputs.emissiveMaterial;
   OUT_COLOR = HOOK_getFinalColor(HOOK_combineColors(c));
+  OUT_COLOR.rgb *= OUT_COLOR.a; // Premultiply alpha before rendering
   HOOK_afterFragment();
 }

test/unit/visual/cases/webgl.js

@@ -345,4 +345,58 @@ visualSuite('WebGL', function() {
       screenshot();
     })
   })
+
+  visualSuite('Opacity', function() {
+    visualTest('Basic colors have opacity applied correctly', (p5, screenshot) => {
+      p5.createCanvas(50, 50, p5.WEBGL);
+      p5.background(255);
+      p5.fill(255, 0, 0, 100);
+      p5.circle(0, 0, 50);
+      screenshot();
+    });
+
+    visualTest('Colors have opacity applied correctly when lights are used', (p5, screenshot) => {
+      p5.createCanvas(50, 50, p5.WEBGL);
+      p5.background(255);
+      p5.ambientLight(255);
+      p5.fill(255, 0, 0, 100);
+      p5.circle(0, 0, 50);
+      screenshot();
+    });
+
+    visualTest('Colors in shader hooks have opacity applied correctly', (p5, screenshot) => {
+      p5.createCanvas(50, 50, p5.WEBGL);
+      const myShader = p5.baseMaterialShader().modify({
+        'Inputs getPixelInputs': `(Inputs inputs) {
+          inputs.color = vec4(1., 0., 0., 100./255.);
+          return inputs;
+        }`
+      })
+      p5.background(255);
+      p5.shader(myShader);
+      p5.circle(0, 0, 50);
+      screenshot();
+    });
+
+    visualTest('Colors in textures have opacity applied correctly', (p5, screenshot) => {
+      p5.createCanvas(50, 50, p5.WEBGL);
+      const tex = p5.createFramebuffer();
+      tex.draw(() => p5.background(255, 0, 0, 100));
+      p5.background(255);
+      p5.texture(tex);
+      p5.circle(0, 0, 50);
+      screenshot();
+    });
+
+    visualTest('Colors in tinted textures have opacity applied correctly', (p5, screenshot) => {
+      p5.createCanvas(50, 50, p5.WEBGL);
+      const tex = p5.createFramebuffer();
+      tex.draw(() => p5.background(255, 0, 0, 255));
+      p5.background(255);
+      p5.texture(tex);
+      p5.tint(255, 100);
+      p5.circle(0, 0, 50);
+      screenshot();
+    });
+  });
 });

test/unit/visual/screenshots/WebGL/Opacity/Basic colors have opacity applied correctly/metadata.json

@@ -0,0 +1,3 @@
+{
+  "numScreenshots": 1
+}

test/unit/visual/screenshots/WebGL/Opacity/Colors have opacity applied correctly when lights are used/metadata.json

@@ -0,0 +1,3 @@
+{
+  "numScreenshots": 1
+}

test/unit/visual/screenshots/WebGL/Opacity/Colors in shader hooks have opacity applied correctly/metadata.json

@@ -0,0 +1,3 @@
+{
+  "numScreenshots": 1
+}

test/unit/visual/screenshots/WebGL/Opacity/Colors in textures have opacity applied correctly/metadata.json

@@ -0,0 +1,3 @@
+{
+  "numScreenshots": 1
+}

test/unit/visual/screenshots/WebGL/Opacity/Colors in tinted textures have opacity applied correctly/metadata.json

@@ -0,0 +1,3 @@
+{
+  "numScreenshots": 1
+}