Q: why does sampler2d always return vec4(0.0) when alpha is zero?

Question

I'm trying to pass some non-texture values to a pixel shader in a texture and i'm running into a weird problem where sampler2d returns vec4(0.0) when the texture's alpha value is zero, regardless of the value of the other 3 bytes.

this isn't a premultiplied alpha thing, or a blending thing, it doesn't happen when the alpha's byte value is 1 through 255, just zero.

if you run the code below you'll see a 2x2 texture in the small 2d canvas being rendered into the large 3d canvas. all 4 pixels have r,g,b = 255. and all 4 pixels have different a values. the top-left pixel of the texture has an alpha value of zero.

the pixel shader sets gl_FragColor.a = 1.0 always.

the reason i don't believe this is a premultiplied alpha thing is that if it were, then surely all 3 pixels would be different shades of grey?

can anyone tell me why this happens?

const cvs = document.getElementById("cvs"),
  {
    width: W,
    height: H
  } = cvs.getBoundingClientRect();
cvs.width = W;
cvs.height = H;


const gl = cvs.getContext("experimental-webgl", {
    premultipliedAlpha: false
  }),
  VERTEX_SHADER = `attribute vec4 a_Position;
    attribute vec2 a_TexCoord;
    varying vec2 v_TexCoord;
    void main() {
      gl_Position = a_Position;
      v_TexCoord = a_TexCoord;
    }`,
  FRAGMENT_SHADER = `precision mediump float;
    uniform sampler2D u_Sampler;
    varying vec2 v_TexCoord;
    void main() {
      gl_FragColor.rgb = texture2D(u_Sampler, v_TexCoord).rgb;
      gl_FragColor.a = 1.0;
    }`,
  vshader = gl.createShader(gl.VERTEX_SHADER),
  fshader = gl.createShader(gl.FRAGMENT_SHADER),
  program = gl.createProgram();

gl.shaderSource(vshader, VERTEX_SHADER);
gl.shaderSource(fshader, FRAGMENT_SHADER);
gl.compileShader(vshader);
gl.compileShader(fshader);
gl.attachShader(program, vshader);
gl.attachShader(program, fshader);
gl.linkProgram(program);
gl.useProgram(program);

gl.clearColor(0.0, 0.0, 0.0, 1.0);
gl.clear(gl.COLOR_BUFFER_BIT);
const farr = new Float32Array([-1, 1, 0, 1, -1, -1, 0, 0,
  1, 1, 1, 1,
  1, -1, 1, 0
]);
const buffer = gl.createBuffer();
gl.bindBuffer(gl.ARRAY_BUFFER, buffer);
gl.bufferData(gl.ARRAY_BUFFER, farr, gl.STATIC_DRAW);
const a_Position = gl.getAttribLocation(program, "a_Position"),
  a_TexCoord = gl.getAttribLocation(program, "a_TexCoord"),
  fsize = farr.BYTES_PER_ELEMENT;
gl.vertexAttribPointer(a_Position, 2, gl.FLOAT, false, 4 * fsize, 0);
gl.vertexAttribPointer(a_TexCoord, 2, gl.FLOAT, false, 4 * fsize, 2 * fsize);
gl.enableVertexAttribArray(a_Position);
gl.enableVertexAttribArray(a_TexCoord);

var image = document.getElementById("img"),
  context = image.getContext("2d"),
  imageData = context.getImageData(0, 0, 2, 2),
  pixels = imageData.data;

for (var i = 0; i < pixels.length; i++) {
  pixels[i] = 255;
}
pixels[0 * 4 + 3] = 0;
pixels[1 * 4 + 3] = 1;
pixels[2 * 4 + 3] = 128;

context.putImageData(imageData, 0, 0, 0, 0, 2, 2);

const texture = gl.createTexture(),
  u_Sampler = gl.getUniformLocation(program, "u_Sampler");

gl.pixelStorei(gl.UNPACK_FLIP_Y_WEBGL, 1);
gl.activeTexture(gl.TEXTURE0);
gl.bindTexture(gl.TEXTURE_2D, texture);
gl.texParameteri(gl.TEXTURE_2D, gl.TEXTURE_MIN_FILTER, gl.NEAREST);
gl.texParameteri(gl.TEXTURE_2D, gl.TEXTURE_MAG_FILTER, gl.NEAREST);
gl.texImage2D(gl.TEXTURE_2D, 0, gl.RGBA, gl.RGBA, gl.UNSIGNED_BYTE, image);

gl.uniform1i(u_Sampler, 0);
gl.clear(gl.COLOR_BUFFER_BIT);
gl.drawArrays(gl.TRIANGLE_STRIP, 0, 4);

html,
body,
canvas {
  padding: 0;
  margin: 0;
}

canvas {
  border: 1px solid red;
  display: block;
  margin: 5px;
}

body {
  background: black;
  display: block;
  position: absolute;
  width: 100%;
  height: 100%;
}

#cvs {
  width: 100px;
  height: 100px;
}

<canvas id="cvs"></canvas>

<canvas id="img" width="2" height="2"></canvas>

2D canvas ALWAYS premultiply rgb values. stackoverflow.com/questions/39744072/… — pleup

gman gman · Accepted Answer · 2018-05-28T12:43:17

What @pleup said. Canvas 2d values are always written into the canvas premultiplied. That means the moment you called putImageData your data was multiplied by alpha and the data was lost.

const ctx = document.createElement("canvas").getContext("2d");
const imgData = ctx.createImageData(2, 2);
const data = imgData.data;
data[ 0] = 255; data[ 3] = 255;
data[ 4] = 255; data[ 7] = 192;
data[ 8] = 255; data[11] = 64;
data[12] = 255; data[15] = 0;

ctx.putImageData(imgData, 0, 0);

const newImgData = ctx.getImageData(0, 0, 2, 2);
const newData = newImgData.data;
console.log(newData[ 0], newData[ 3]);
console.log(newData[ 4], newData[ 7]);
console.log(newData[ 8], newData[11]);
console.log(newData[12], newData[15]);

Using 255 for color partly hides the issue since what's in the 2d canvas after you call putImageData is

red = 255   alpha = 255
red = 192   alpha = 192
red =  64   alpha = 64
red =   0   alpha = 0

Unpremultiplying (when uploading to WebGL) you get 255s back for all values except 0

                           red        alpha  result
red = 255   alpha = 255  = 255 * 255 / 255 = 255 
red = 192   alpha = 192  = 192 * 255 / 192 = 255
red =  64   alpha = 64   =  64 * 255 / 64  = 255
red =   0   alpha = 0    =   0 * 0         =   0

If the color value was say 20 and different alphas

red = 20 *  10 / 255 = 1   alpha = 10
red = 20 *   7 / 255 = 1   alpha = 7
red = 20 *   4 / 255 = 0   alpha = 4
red = 20 *   0 / 255 = 0   alpha = 0

And then unpremultiplying gets

 1 * 255 / 10  = 26    not even close to what we put in
 1 * 255 /  7  = 36    not even close to what we put in
 0 * 255 /  4  = 0
 0 * 0         = 0

Just pointing out how lossy it is.

const ctx = document.createElement("canvas").getContext("2d");
const imgData = ctx.createImageData(2, 2);
const data = imgData.data;
data[ 0] = 20; data[ 3] = 10;
data[ 4] = 20; data[ 7] = 7;
data[ 8] = 20; data[11] = 4;
data[12] = 20; data[15] = 0;

ctx.putImageData(imgData, 0, 0);

const newImgData = ctx.getImageData(0, 0, 2, 2);
const newData = newImgData.data;
console.log(newData[ 0], newData[ 3]);
console.log(newData[ 4], newData[ 7]);
console.log(newData[ 8], newData[11]);
console.log(newData[12], newData[15]);

If you actually want to manually put data in a texture in WebGL you should just use a typedArray

const width = 2;
const height = 2;
const data = new Uint8Array(width * height * 4);

data[ 0] = 255; data[ 3] = 255;
data[ 4] = 255; data[ 7] = 192;
data[ 8] = 255; data[11] = 64;
data[12] = 255; data[15] = 0;

const level = 0;
const internalFormat = gl.RGBA;
const border = 0;
const format = gl.RGBA;
const type = gl.UNSIGNED_BYTE;
gl.texImage2D(gl.TEXTURE_2D, level, internalFormat, width, height, border,
              format, type, data);

If you want to read that data instead of going through a 2d canvas just call gl.readPixels

const newData = new Uint8Array(width * height * 4);
gl.readPixels(0, 0, width, height, format, type);

const gl = document.querySelector("canvas").getContext("webgl", {
  premultipliedAlpha: false,
});
const VERTEX_SHADER = `
  attribute vec4 a_Position;
  attribute vec2 a_TexCoord;
  varying vec2 v_TexCoord;
  void main() {
    gl_Position = a_Position;
    v_TexCoord = a_TexCoord;
  }
`;
const FRAGMENT_SHADER = `
  precision mediump float;
  uniform sampler2D u_Sampler;
  varying vec2 v_TexCoord;
  void main() {
    gl_FragColor = texture2D(u_Sampler, v_TexCoord);
  }
`;
const vshader = gl.createShader(gl.VERTEX_SHADER);
const fshader = gl.createShader(gl.FRAGMENT_SHADER);
const program = gl.createProgram();

gl.shaderSource(vshader, VERTEX_SHADER);
gl.shaderSource(fshader, FRAGMENT_SHADER);
gl.compileShader(vshader);
gl.compileShader(fshader);
gl.attachShader(program, vshader);
gl.attachShader(program, fshader);
gl.linkProgram(program);
gl.useProgram(program);

gl.clearColor(0.0, 0.0, 0.0, 1.0);
gl.clear(gl.COLOR_BUFFER_BIT);

const farr = new Float32Array([
  -1, 1, 0, 1, 
  -1, -1, 0, 0,
  1, 1, 1, 1,
  1, -1, 1, 0
]);
const buffer = gl.createBuffer();
gl.bindBuffer(gl.ARRAY_BUFFER, buffer);
gl.bufferData(gl.ARRAY_BUFFER, farr, gl.STATIC_DRAW);
const a_Position = gl.getAttribLocation(program, "a_Position");
const a_TexCoord = gl.getAttribLocation(program, "a_TexCoord");
const fsize = farr.BYTES_PER_ELEMENT;
gl.vertexAttribPointer(a_Position, 2, gl.FLOAT, false, 4 * fsize, 0);
gl.vertexAttribPointer(a_TexCoord, 2, gl.FLOAT, false, 4 * fsize, 2 * fsize);
gl.enableVertexAttribArray(a_Position);
gl.enableVertexAttribArray(a_TexCoord);

const pixels = new Uint8Array(2 * 2 * 4);
pixels.fill(255);
pixels[0 * 4 + 3] = 0;
pixels[1 * 4 + 3] = 1;
pixels[2 * 4 + 3] = 128;
const texture = gl.createTexture();
const u_Sampler = gl.getUniformLocation(program, "u_Sampler");

gl.bindTexture(gl.TEXTURE_2D, texture);
gl.texParameteri(gl.TEXTURE_2D, gl.TEXTURE_MIN_FILTER, gl.NEAREST);
gl.texParameteri(gl.TEXTURE_2D, gl.TEXTURE_MAG_FILTER, gl.NEAREST);
gl.texImage2D(gl.TEXTURE_2D, 0, gl.RGBA, 2, 2, 0, gl.RGBA, gl.UNSIGNED_BYTE, pixels);

gl.drawArrays(gl.TRIANGLE_STRIP, 0, 4);

canvas { background: red; }

<canvas></canvas>

Q: why does sampler2d always return vec4(0.0) when alpha is zero?

1 Answers