Use premultiplied alpha everywhere, take 2

src/BitmapSkin.js

@@ -18,9 +18,6 @@ class BitmapSkin extends Skin {
         /** @type {!RenderWebGL} */
         this._renderer = renderer;
 
-        /** @type {WebGLTexture} */
-        this._texture = null;
-
         /** @type {Array<int>} */
         this._textureSize = [0, 0];
     }
@@ -95,22 +92,17 @@ class BitmapSkin extends Skin {
             textureData = context.getImageData(0, 0, bitmapData.width, bitmapData.height);
         }
 
-        if (this._texture) {
-            gl.bindTexture(gl.TEXTURE_2D, this._texture);
-            gl.texImage2D(gl.TEXTURE_2D, 0, gl.RGBA, gl.RGBA, gl.UNSIGNED_BYTE, textureData);
-            this._silhouette.update(textureData);
-        } else {
-            // TODO: mipmaps?
+        if (this._texture === null) {
             const textureOptions = {
-                auto: true,
-                wrap: gl.CLAMP_TO_EDGE,
-                src: textureData
+                auto: false,
+                wrap: gl.CLAMP_TO_EDGE
             };
 
             this._texture = twgl.createTexture(gl, textureOptions);
-            this._silhouette.update(textureData);
         }
 
+        this._setTexture(textureData);
+
         // Do these last in case any of the above throws an exception
         this._costumeResolution = costumeResolution || 2;
         this._textureSize = BitmapSkin._getBitmapSize(bitmapData);

src/Drawable.js

@@ -685,6 +685,9 @@ class Drawable {
         const localPosition = getLocalPosition(drawable, vec);
         if (localPosition[0] < 0 || localPosition[1] < 0 ||
             localPosition[0] > 1 || localPosition[1] > 1) {
+            dst[0] = 0;
+            dst[1] = 0;
+            dst[2] = 0;
             dst[3] = 0;
             return dst;
         }

src/EffectTransform.js

@@ -130,15 +130,21 @@ class EffectTransform {
         const effects = drawable.enabledEffects;
         const uniforms = drawable.getUniforms();
 
-        if ((effects & ShaderManager.EFFECT_INFO.ghost.mask) !== 0) {
-            // gl_FragColor.a *= u_ghost
-            inOutColor[3] *= uniforms.u_ghost;
-        }
-
         const enableColor = (effects & ShaderManager.EFFECT_INFO.color.mask) !== 0;
         const enableBrightness = (effects & ShaderManager.EFFECT_INFO.brightness.mask) !== 0;
 
         if (enableColor || enableBrightness) {
+            // gl_FragColor.rgb /= gl_FragColor.a + epsilon;
+            // Here, we're dividing by the (previously pre-multiplied) alpha to ensure HSV is properly calculated
+            // for partially transparent pixels.
+            // epsilon is present in the shader because dividing by 0 (fully transparent pixels) messes up calculations.
+            // We're doing this with a Uint8ClampedArray here, so dividing by 0 just gives 255. We're later multiplying
+            // by 0 again, so it won't affect results.
+            const alpha = inOutColor[3] / 255;
+            inOutColor[0] /= alpha;
+            inOutColor[1] /= alpha;
+            inOutColor[2] /= alpha;
+
             // vec3 hsl = convertRGB2HSL(gl_FragColor.xyz);
             const hsl = rgbToHsl(inOutColor);
 
@@ -171,6 +177,20 @@ class EffectTransform {
             }
             // gl_FragColor.rgb = convertHSL2RGB(hsl);
             inOutColor.set(hslToRgb(hsl));
+
+            // gl_FragColor.rgb *= gl_FragColor.a + epsilon;
+            // Now we're doing the reverse, premultiplying by the alpha once again.
+            inOutColor[0] *= alpha;
+            inOutColor[1] *= alpha;
+            inOutColor[2] *= alpha;
+        }
+
+        if ((effects & ShaderManager.EFFECT_INFO.ghost.mask) !== 0) {
+            // gl_FragColor *= u_ghost
+            inOutColor[0] *= uniforms.u_ghost;
+            inOutColor[1] *= uniforms.u_ghost;
+            inOutColor[2] *= uniforms.u_ghost;
+            inOutColor[3] *= uniforms.u_ghost;
         }
 
         return inOutColor;

src/PenSkin.js

@@ -25,6 +25,12 @@ const DefaultPenAttributes = {
     diameter: 1
 };
 
+/**
+ * Reused memory location for storing a premultiplied pen color.
+ * @type {FloatArray}
+ */
+const __premultipliedColor = [0, 0, 0, 0];
+
 
 /**
  * Reused memory location for projection matrices.
@@ -88,9 +94,6 @@ class PenSkin extends Skin {
         /** @type {HTMLCanvasElement} */
         this._canvas = document.createElement('canvas');
 
-        /** @type {WebGLTexture} */
-        this._texture = null;
-
         /** @type {WebGLTexture} */
         this._exportTexture = null;
 
@@ -123,7 +126,7 @@ class PenSkin extends Skin {
 
         const NO_EFFECTS = 0;
         /** @type {twgl.ProgramInfo} */
-        this._stampShader = this._renderer._shaderManager.getShader(ShaderManager.DRAW_MODE.stamp, NO_EFFECTS);
+        this._stampShader = this._renderer._shaderManager.getShader(ShaderManager.DRAW_MODE.default, NO_EFFECTS);
 
         /** @type {twgl.ProgramInfo} */
         this._lineShader = this._renderer._shaderManager.getShader(ShaderManager.DRAW_MODE.lineSample, NO_EFFECTS);
@@ -154,13 +157,6 @@ class PenSkin extends Skin {
         return true;
     }
 
-    /**
-     * @returns {boolean} true if alpha is premultiplied, false otherwise
-     */
-    get hasPremultipliedAlpha () {
-        return true;
-    }
-
     /**
      * @return {Array<number>} the "native" size, in texels, of this skin. [width, height]
      */
@@ -188,7 +184,7 @@ class PenSkin extends Skin {
     clear () {
         const gl = this._renderer.gl;
         twgl.bindFramebufferInfo(gl, this._framebuffer);
-        
+
         /* Reset framebuffer to transparent black */
         gl.clearColor(0, 0, 0, 0);
         gl.clear(gl.COLOR_BUFFER_BIT);
@@ -317,13 +313,6 @@ class PenSkin extends Skin {
 
         twgl.bindFramebufferInfo(gl, this._framebuffer);
 
-        // Needs a blend function that blends a destination that starts with
-        // no alpha.
-        gl.blendFuncSeparate(
-            gl.SRC_ALPHA, gl.ONE_MINUS_SRC_ALPHA,
-            gl.ONE, gl.ONE_MINUS_SRC_ALPHA
-        );
-
         gl.viewport(0, 0, bounds.width, bounds.height);
 
         gl.useProgram(currentShader.program);
@@ -344,8 +333,6 @@ class PenSkin extends Skin {
     _exitDrawLineOnBuffer () {
         const gl = this._renderer.gl;
 
-        gl.blendFuncSeparate(gl.ONE, gl.ONE_MINUS_SRC_ALPHA, gl.ZERO, gl.ONE);
-
         twgl.bindFramebufferInfo(gl, null);
     }
 
@@ -384,6 +371,13 @@ class PenSkin extends Skin {
         const radius = diameter / 2;
         const yScalar = (0.50001 - (radius / (length + diameter)));
 
+        // Premultiply pen color by pen transparency
+        const penColor = penAttributes.color4f || DefaultPenAttributes.color4f;
+        __premultipliedColor[0] = penColor[0] * penColor[3];
+        __premultipliedColor[1] = penColor[1] * penColor[3];
+        __premultipliedColor[2] = penColor[2] * penColor[3];
+        __premultipliedColor[3] = penColor[3];
+
         const uniforms = {
             u_positionScalar: yScalar,
             u_capScale: diameter,
@@ -397,7 +391,7 @@ class PenSkin extends Skin {
                 twgl.m4.scaling(scalingVector, __modelScalingMatrix),
                 __modelMatrix
             ),
-            u_lineColor: penAttributes.color4f || DefaultPenAttributes.color4f
+            u_lineColor: __premultipliedColor
         };
 
         twgl.setUniforms(currentShader, uniforms);
@@ -490,8 +484,6 @@ class PenSkin extends Skin {
 
         twgl.bindFramebufferInfo(gl, this._framebuffer);
 
-        gl.blendFuncSeparate(gl.SRC_ALPHA, gl.ONE_MINUS_SRC_ALPHA, gl.ONE, gl.ONE_MINUS_SRC_ALPHA);
-
         this._drawRectangleRegionEnter(this._stampShader, this._bounds);
     }
 
@@ -501,8 +493,6 @@ class PenSkin extends Skin {
     _exitDrawToBuffer () {
         const gl = this._renderer.gl;
 
-        gl.blendFuncSeparate(gl.ONE, gl.ONE_MINUS_SRC_ALPHA, gl.ZERO, gl.ONE);
-
         twgl.bindFramebufferInfo(gl, null);
     }
 
@@ -661,7 +651,7 @@ class PenSkin extends Skin {
             skinImageData.data.set(skinPixels);
             skinContext.putImageData(skinImageData, 0, 0);
 
-            this._silhouette.update(this._canvas);
+            this._silhouette.update(this._canvas, true /* isPremultiplied */);
 
             this._silhouetteDirty = false;
         }

src/RenderWebGL.js

@@ -196,7 +196,7 @@ class RenderWebGL extends EventEmitter {
         gl.disable(gl.DEPTH_TEST);
         /** @todo disable when no partial transparency? */
         gl.enable(gl.BLEND);
-        gl.blendFuncSeparate(gl.ONE, gl.ONE_MINUS_SRC_ALPHA, gl.ZERO, gl.ONE);
+        gl.blendFunc(gl.ONE, gl.ONE_MINUS_SRC_ALPHA);
     }
 
     /**
@@ -834,7 +834,8 @@ class RenderWebGL extends EventEmitter {
                 projection,
                 {
                     extraUniforms,
-                    ignoreVisibility: true // Touching color ignores sprite visibility
+                    ignoreVisibility: true, // Touching color ignores sprite visibility,
+                    effectMask: ~ShaderManager.EFFECT_INFO.ghost.mask
                 });
 
             gl.stencilFunc(gl.EQUAL, 1, 1);
@@ -1554,7 +1555,7 @@ class RenderWebGL extends EventEmitter {
         const projection = twgl.m4.ortho(bounds.left, bounds.right, bounds.top, bounds.bottom, -1, 1);
 
         // Draw the stamped sprite onto the PenSkin's framebuffer.
-        this._drawThese([stampID], ShaderManager.DRAW_MODE.stamp, projection, {ignoreVisibility: true});
+        this._drawThese([stampID], ShaderManager.DRAW_MODE.default, projection, {ignoreVisibility: true});
         skin._silhouetteDirty = true;
     }
 
@@ -1744,14 +1745,6 @@ class RenderWebGL extends EventEmitter {
             }
 
             twgl.setUniforms(currentShader, uniforms);
-
-            /* adjust blend function for this skin */
-            if (drawable.skin.hasPremultipliedAlpha){
-                gl.blendFuncSeparate(gl.ONE, gl.ONE_MINUS_SRC_ALPHA, gl.ONE, gl.ONE_MINUS_SRC_ALPHA);
-            } else {
-                gl.blendFuncSeparate(gl.SRC_ALPHA, gl.ONE_MINUS_SRC_ALPHA, gl.ONE, gl.ONE_MINUS_SRC_ALPHA);
-            }
-
             twgl.drawBufferInfo(gl, this._bufferInfo, gl.TRIANGLES);
         }
 
@@ -1902,13 +1895,11 @@ class RenderWebGL extends EventEmitter {
             }
             */
             Drawable.sampleColor4b(vec, drawables[index].drawable, __blendColor);
-            // if we are fully transparent, go to the next one "down"
-            const sampleAlpha = __blendColor[3] / 255;
-            // premultiply alpha
-            dst[0] += __blendColor[0] * blendAlpha * sampleAlpha;
-            dst[1] += __blendColor[1] * blendAlpha * sampleAlpha;
-            dst[2] += __blendColor[2] * blendAlpha * sampleAlpha;
-            blendAlpha *= (1 - sampleAlpha);
+            // Equivalent to gl.blendFunc(gl.ONE, gl.ONE_MINUS_SRC_ALPHA)
+            dst[0] += __blendColor[0] * blendAlpha;
+            dst[1] += __blendColor[1] * blendAlpha;
+            dst[2] += __blendColor[2] * blendAlpha;
+            blendAlpha *= (1 - (__blendColor[3] / 255));
         }
         // Backdrop could be transparent, so we need to go to the "clear color" of the
         // draw scene (white) as a fallback if everything was alpha

src/SVGSkin.js

@@ -90,7 +90,8 @@ class SVGSkin extends Skin {
         const textureOptions = {
             auto: false,
             wrap: this._renderer.gl.CLAMP_TO_EDGE,
-            src: textureData
+            src: textureData,
+            premultiplyAlpha: true
         };
 
         const mip = twgl.createTexture(this._renderer.gl, textureOptions);

src/ShaderManager.js

@@ -171,12 +171,7 @@ ShaderManager.DRAW_MODE = {
     /**
      * Sample a "texture" to draw a line with caps.
      */
-    lineSample: 'lineSample',
-
-    /**
-     * Draw normally except for pre-multiplied alpha
-     */
-    stamp: 'stamp'
+    lineSample: 'lineSample'
 };
 
 module.exports = ShaderManager;

src/Silhouette.js

@@ -20,7 +20,7 @@ let __SilhouetteUpdateCanvas;
  * @return {number} Alpha value for x/y position
  */
 const getPoint = ({_width: width, _height: height, _colorData: data}, x, y) => {
-    // 0 if outside bouds, otherwise read from data.
+    // 0 if outside bounds, otherwise read from data.
     if (x >= width || y >= height || x < 0 || y < 0) {
         return 0;
     }
@@ -39,14 +39,39 @@ const __cornerWork = [
 
 /**
  * Get the color from a given silhouette at an x/y local texture position.
+ * Multiply color values by alpha for proper blending.
  * @param {Silhouette} The silhouette to sample.
  * @param {number} x X position of texture (0-1).
  * @param {number} y Y position of texture (0-1).
  * @param {Uint8ClampedArray} dst A color 4b space.
  * @return {Uint8ClampedArray} The dst vector.
  */
 const getColor4b = ({_width: width, _height: height, _colorData: data}, x, y, dst) => {
-    // 0 if outside bouds, otherwise read from data.
+    // 0 if outside bounds, otherwise read from data.
+    if (x >= width || y >= height || x < 0 || y < 0) {
+        return dst.fill(0);
+    }
+    const offset = ((y * width) + x) * 4;
+    // premultiply alpha
+    const alpha = data[offset + 3] / 255;
+    dst[0] = data[offset] * alpha;
+    dst[1] = data[offset + 1] * alpha;
+    dst[2] = data[offset + 2] * alpha;
+    dst[3] = data[offset + 3];
+    return dst;
+};
+
+/**
+ * Get the color from a given silhouette at an x/y local texture position.
+ * Do not multiply color values by alpha, as it has already been done.
+ * @param {Silhouette} The silhouette to sample.
+ * @param {number} x X position of texture (0-1).
+ * @param {number} y Y position of texture (0-1).
+ * @param {Uint8ClampedArray} dst A color 4b space.
+ * @return {Uint8ClampedArray} The dst vector.
+ */
+const getPremultipliedColor4b = ({_width: width, _height: height, _colorData: data}, x, y, dst) => {
+    // 0 if outside bounds, otherwise read from data.
     if (x >= width || y >= height || x < 0 || y < 0) {
         return dst.fill(0);
     }
@@ -78,15 +103,21 @@ class Silhouette {
          */
         this._colorData = null;
 
+        // By default, silhouettes are assumed not to contain premultiplied image data,
+        // so when we get a color, we want to multiply it by its alpha channel.
+        // Point `_getColor` to the version of the function that multiplies.
+        this._getColor = getColor4b;
+
         this.colorAtNearest = this.colorAtLinear = (_, dst) => dst.fill(0);
     }
 
     /**
      * Update this silhouette with the bitmapData for a skin.
-     * @param {*} bitmapData An image, canvas or other element that the skin
+     * @param {ImageData|HTMLCanvasElement|HTMLImageElement} bitmapData An image, canvas or other element that the skin
+     * @param {boolean} isPremultiplied True if the source bitmap data comes premultiplied (e.g. from readPixels).
      * rendering can be queried from.
      */
-    update (bitmapData) {
+    update (bitmapData, isPremultiplied = false) {
         let imageData;
         if (bitmapData instanceof ImageData) {
             // If handed ImageData directly, use it directly.
@@ -109,6 +140,12 @@ class Silhouette {
             imageData = ctx.getImageData(0, 0, width, height);
         }
 
+        if (isPremultiplied) {
+            this._getColor = getPremultipliedColor4b;
+        } else {
+            this._getColor = getColor4b;
+        }
+
         this._colorData = imageData.data;
         // delete our custom overriden "uninitalized" color functions
         // let the prototype work for itself
@@ -124,7 +161,7 @@ class Silhouette {
      * @returns {Uint8ClampedArray} dst
      */
     colorAtNearest (vec, dst) {
-        return getColor4b(
+        return this._getColor(
             this,
             Math.floor(vec[0] * (this._width - 1)),
             Math.floor(vec[1] * (this._height - 1)),
@@ -151,10 +188,10 @@ class Silhouette {
         const xFloor = Math.floor(x);
         const yFloor = Math.floor(y);
 
-        const x0y0 = getColor4b(this, xFloor, yFloor, __cornerWork[0]);
-        const x1y0 = getColor4b(this, xFloor + 1, yFloor, __cornerWork[1]);
-        const x0y1 = getColor4b(this, xFloor, yFloor + 1, __cornerWork[2]);
-        const x1y1 = getColor4b(this, xFloor + 1, yFloor + 1, __cornerWork[3]);
+        const x0y0 = this._getColor(this, xFloor, yFloor, __cornerWork[0]);
+        const x1y0 = this._getColor(this, xFloor + 1, yFloor, __cornerWork[1]);
+        const x0y1 = this._getColor(this, xFloor, yFloor + 1, __cornerWork[2]);
+        const x1y1 = this._getColor(this, xFloor + 1, yFloor + 1, __cornerWork[3]);
 
         dst[0] = (x0y0[0] * x0D * y0D) + (x0y1[0] * x0D * y1D) + (x1y0[0] * x1D * y0D) + (x1y1[0] * x1D * y1D);
         dst[1] = (x0y0[1] * x0D * y0D) + (x0y1[1] * x0D * y1D) + (x1y0[1] * x1D * y0D) + (x1y1[1] * x1D * y1D);

src/Skin.js

@@ -33,6 +33,9 @@ class Skin extends EventEmitter {
         /** @type {Vec3} */
         this._rotationCenter = twgl.v3.create(0, 0);
 
+        /** @type {WebGLTexture} */
+        this._texture = null;
+
         /**
          * The uniforms to be used by the vertex and pixel shaders.
          * Some of these are used by other parts of the renderer as well.
@@ -76,13 +79,6 @@ class Skin extends EventEmitter {
         return false;
     }
 
-    /**
-     * @returns {boolean} true if alpha is premultiplied, false otherwise
-     */
-    get hasPremultipliedAlpha () {
-        return false;
-    }
-
     /**
      * @return {int} the unique ID for this Skin.
      */
@@ -171,6 +167,23 @@ class Skin extends EventEmitter {
      */
     updateSilhouette () {}
 
+    /**
+     * Set this skin's texture to the given image.
+     * @param {ImageData|HTMLCanvasElement} textureData - The canvas or image data to set the texture to.
+     */
+    _setTexture (textureData) {
+        const gl = this._renderer.gl;
+
+        gl.bindTexture(gl.TEXTURE_2D, this._texture);
+        // Premultiplied alpha is necessary for proper blending.
+        // See http://www.realtimerendering.com/blog/gpus-prefer-premultiplication/
+        gl.pixelStorei(gl.UNPACK_PREMULTIPLY_ALPHA_WEBGL, true);
+        gl.texImage2D(gl.TEXTURE_2D, 0, gl.RGBA, gl.RGBA, gl.UNSIGNED_BYTE, textureData);
+        gl.pixelStorei(gl.UNPACK_PREMULTIPLY_ALPHA_WEBGL, false);
+
+        this._silhouette.update(textureData);
+    }
+
     /**
      * Set the contents of this skin to an empty skin.
      * @fires Skin.event:WasAltered

src/TextBubbleSkin.js

@@ -42,9 +42,6 @@ class TextBubbleSkin extends Skin {
         /** @type {HTMLCanvasElement} */
         this._canvas = document.createElement('canvas');
 
-        /** @type {WebGLTexture} */
-        this._texture = null;
-
         /** @type {Array<number>} */
         this._size = [0, 0];
 
@@ -272,9 +269,7 @@ class TextBubbleSkin extends Skin {
                 this._texture = twgl.createTexture(gl, textureOptions);
             }
 
-            gl.bindTexture(gl.TEXTURE_2D, this._texture);
-            gl.texImage2D(gl.TEXTURE_2D, 0, gl.RGBA, gl.RGBA, gl.UNSIGNED_BYTE, textureData);
-            this._silhouette.update(textureData);
+            this._setTexture(textureData);
         }
 
         return this._texture;

src/shaders/sprite.frag

@@ -43,14 +43,16 @@ uniform sampler2D u_skin;
 
 varying vec2 v_texCoord;
 
+// Add this to divisors to prevent division by 0, which results in NaNs propagating through calculations.
+// Smaller values can cause problems on some mobile devices.
+const float epsilon = 1e-3;
+
 #if !defined(DRAW_MODE_silhouette) && (defined(ENABLE_color))
 // Branchless color conversions based on code from:
 // http://www.chilliant.com/rgb2hsv.html by Ian Taylor
 // Based in part on work by Sam Hocevar and Emil Persson
 // See also: https://en.wikipedia.org/wiki/HSL_and_HSV#Formal_derivation
 
-// Smaller values can cause problems on some mobile devices
-const float epsilon = 1e-3;
 
 // Convert an RGB color to Hue, Saturation, and Value.
 // All components of input and output are expected to be in the [0,1] range.
@@ -153,16 +155,12 @@ void main()
 
 	gl_FragColor = texture2D(u_skin, texcoord0);
 
-    #ifdef ENABLE_ghost
-    gl_FragColor.a *= u_ghost;
-    #endif // ENABLE_ghost
+	#if defined(ENABLE_color) || defined(ENABLE_brightness)
+	// Divide premultiplied alpha values for proper color processing
+	// Add epsilon to avoid dividing by 0 for fully transparent pixels
+	gl_FragColor.rgb = clamp(gl_FragColor.rgb / (gl_FragColor.a + epsilon), 0.0, 1.0);
 
-	#ifdef DRAW_MODE_silhouette
-	// switch to u_silhouetteColor only AFTER the alpha test
-	gl_FragColor = u_silhouetteColor;
-	#else // DRAW_MODE_silhouette
-
-	#if defined(ENABLE_color)
+	#ifdef ENABLE_color
 	{
 		vec3 hsv = convertRGB2HSV(gl_FragColor.xyz);
 
@@ -178,11 +176,29 @@ void main()
 
 		gl_FragColor.rgb = convertHSV2RGB(hsv);
 	}
-	#endif // defined(ENABLE_color)
+	#endif // ENABLE_color
 
-	#if defined(ENABLE_brightness)
+	#ifdef ENABLE_brightness
 	gl_FragColor.rgb = clamp(gl_FragColor.rgb + vec3(u_brightness), vec3(0), vec3(1));
-	#endif // defined(ENABLE_brightness)
+	#endif // ENABLE_brightness
+
+	// Re-multiply color values
+	gl_FragColor.rgb *= gl_FragColor.a + epsilon;
+
+	#endif // defined(ENABLE_color) || defined(ENABLE_brightness)
+
+	#ifdef ENABLE_ghost
+	gl_FragColor *= u_ghost;
+	#endif // ENABLE_ghost
+
+	#ifdef DRAW_MODE_silhouette
+	// Discard fully transparent pixels for stencil test
+	if (gl_FragColor.a == 0.0) {
+		discard;
+	}
+	// switch to u_silhouetteColor only AFTER the alpha test
+	gl_FragColor = u_silhouetteColor;
+	#else // DRAW_MODE_silhouette
 
 	#ifdef DRAW_MODE_colorMask
 	vec3 maskDistance = abs(gl_FragColor.rgb - u_colorMask);
@@ -195,8 +211,7 @@ void main()
 	#endif // DRAW_MODE_silhouette
 
 	#else // DRAW_MODE_lineSample
-	gl_FragColor = u_lineColor;
-	gl_FragColor.a *= clamp(
+	gl_FragColor = u_lineColor * clamp(
 		// Scale the capScale a little to have an aliased region.
 		(u_capScale + u_aliasAmount -
 			u_capScale * 2.0 * distance(v_texCoord, vec2(0.5, 0.5))