opengl-es - 如何在 JavaScript 中为 WebGL2 打包/解包 11 位和 10 位浮点数

Question

WebGL2 支持UNSIGNED_INT_10F_11F_11F_REV纹理类型，其中浮点值表示为无符号 10 位和 11 位浮点数。如何创建打包的值以及如何将它们解包回浮点数？

Answer 1

这里有一些功能

// Adapted from https://gamedev.stackexchange.com/a/17329/79
var to11uf = (function() {
  const F11_EXPONENT_BITS = 0x1F
  const F11_EXPONENT_SHIFT = 6
  const F11_EXPONENT_BIAS = 15
  const F11_MANTISSA_BITS = 0x3f
  const F11_MANTISSA_SHIFT = (23 - F11_EXPONENT_SHIFT)
  const F11_MAX_EXPONENT = (F11_EXPONENT_BITS << F11_EXPONENT_SHIFT)

  const floatView = new Float32Array(1);
  const int32View = new Int32Array(floatView.buffer);

  return function(val) {
    floatView[0] = val;
    let f32 = int32View[0];
    let f11 = 0;
    /* Decode IEEE 754 little-endian 32-bit floating-point value */
    if (f32 & 0x80000000) {
      // negative values go to 0
      return 0;
    }
    /* Map exponent to the range [-127,128] */
    let exponent = ((f32 >> 23) & 0xff) - 127;
    let mantissa = f32 & 0x007fffff;
    if (exponent == 128) {
      /* Infinity or NaN */
      f11 = F11_MAX_EXPONENT;
      if (mantissa) {
        f11 |= (mantissa & F11_MANTISSA_BITS);
      }
    } else if (exponent > 15) {
      /* Overflow - flush to Infinity */
      f11 = F11_MAX_EXPONENT;
    } else if (exponent > -15) {
      /* Representable value */
      exponent += F11_EXPONENT_BIAS;
      mantissa >>= F11_MANTISSA_SHIFT;
      f11 = exponent << F11_EXPONENT_SHIFT | mantissa;
    } else {
        f11 = 0;
    }
    return f11;
  }
}());

// Adapted from http://gamedev.stackexchange.com/a/17329/79
var to10uf = (function() {
  const F10_EXPONENT_BITS = 0x1F
  const F10_EXPONENT_SHIFT = 5
  const F10_EXPONENT_BIAS = 15
  const F10_MANTISSA_BITS = 0x1f
  const F10_MANTISSA_SHIFT = (23 - F10_EXPONENT_SHIFT)
  const F10_MAX_EXPONENT = (F10_EXPONENT_BITS << F10_EXPONENT_SHIFT)

  const floatView = new Float32Array(1);
  const int32View = new Int32Array(floatView.buffer);

  return function(val) {
    floatView[0] = val;
    let f32 = int32View[0];
    let f10 = 0;
    /* Decode IEEE 754 little-endian 32-bit floating-point value */
    if (f32 & 0x80000000) {
      // negative values go to 0
      return 0;
    }
    /* Map exponent to the range [-127,128] */
    let exponent = ((f32 >> 23) & 0xff) - 127;
    let mantissa = f32 & 0x007fffff;
    if (exponent == 128) {
      /* Infinity or NaN */
      f10 = F10_MAX_EXPONENT;
      if (mantissa) {
        f10 |= (mantissa & F10_MANTISSA_BITS);
      }
    } else if (exponent > 15) {
      /* Overflow - flush to Infinity */
      f10 = F10_MAX_EXPONENT;
    } else if (exponent > -15) {
      /* Representable value */
      exponent += F10_EXPONENT_BIAS;
      mantissa >>= F10_MANTISSA_SHIFT;
      f10 = exponent << F10_EXPONENT_SHIFT | mantissa;
    } else {
        f10 = 0;
    }
    return f10;
  }
}());


// From OpenGL ES 3.0 spec 2.1.3
function from11uf(v) {
  const e = v >> 6;
  const m = v & 0x3F;
  if (e === 0) {
    if (m === 0) {
      return 0;
    } else {
      return Math.pow(2, -14) * (m / 64);
    }
  } else {
    if (e < 31) {
      return Math.pow(2, e - 15) * (1 + m / 64);
    } else {
      if (m === 0) {
        return 0;  // Inf
      } else {
        return 0;  // Nan
      }
    }
  }
}

// From OpenGL ES 3.0 spec 2.1.4
function from10uf(v) {
  const e = v >> 5;
  const m = v & 0x1F;
  if (e === 0) {
    if (m === 0) {
      return 0;
    } else {
      return Math.pow(2, -14) * (m / 32);
    }
  } else {
    if (e < 31) {
      return Math.pow(2, e - 15) * (1 + m / 32);
    } else {
      if (m === 0) {
        return 0;  // Inf
      } else {
        return 0;  // Nan
      }
    }
  }
}

function rgb101111(r, g, b) {
  return (to11uf(r) <<  0) |
         (to11uf(g) << 11) |
         (to10uf(b) << 22);
}

和一个例子

// Adapted from https://gamedev.stackexchange.com/a/17329/79
var to11uf = (function() {
  const F11_EXPONENT_BITS = 0x1F
  const F11_EXPONENT_SHIFT = 6
  const F11_EXPONENT_BIAS = 15
  const F11_MANTISSA_BITS = 0x3f
  const F11_MANTISSA_SHIFT = (23 - F11_EXPONENT_SHIFT)
  const F11_MAX_EXPONENT = (F11_EXPONENT_BITS << F11_EXPONENT_SHIFT)

  const floatView = new Float32Array(1);
  const int32View = new Int32Array(floatView.buffer);

  return function(val) {
    floatView[0] = val;
    let f32 = int32View[0];
    let f11 = 0;
    /* Decode IEEE 754 little-endian 32-bit floating-point value */
    if (f32 & 0x80000000) {
      // negative values go to 0
      return 0;
    }
    /* Map exponent to the range [-127,128] */
    let exponent = ((f32 >> 23) & 0xff) - 127;
    let mantissa = f32 & 0x007fffff;
    if (exponent == 128) {
      /* Infinity or NaN */
      f11 = F11_MAX_EXPONENT;
      if (mantissa) {
        f11 |= (mantissa & F11_MANTISSA_BITS);
      }
    } else if (exponent > 15) {
      /* Overflow - flush to Infinity */
      f11 = F11_MAX_EXPONENT;
    } else if (exponent > -15) {
      /* Representable value */
      exponent += F11_EXPONENT_BIAS;
      mantissa >>= F11_MANTISSA_SHIFT;
      f11 = exponent << F11_EXPONENT_SHIFT | mantissa;
    } else {
        f11 = 0;
    }
    return f11;
  }
}());

// Adapted from http://gamedev.stackexchange.com/a/17329/79
var to10uf = (function() {
  const F10_EXPONENT_BITS = 0x1F
  const F10_EXPONENT_SHIFT = 5
  const F10_EXPONENT_BIAS = 15
  const F10_MANTISSA_BITS = 0x1f
  const F10_MANTISSA_SHIFT = (23 - F10_EXPONENT_SHIFT)
  const F10_MAX_EXPONENT = (F10_EXPONENT_BITS << F10_EXPONENT_SHIFT)

  const floatView = new Float32Array(1);
  const int32View = new Int32Array(floatView.buffer);

  return function(val) {
    floatView[0] = val;
    let f32 = int32View[0];
    let f10 = 0;
    /* Decode IEEE 754 little-endian 32-bit floating-point value */
    if (f32 & 0x80000000) {
      // negative values go to 0
      return 0;
    }
    /* Map exponent to the range [-127,128] */
    let exponent = ((f32 >> 23) & 0xff) - 127;
    let mantissa = f32 & 0x007fffff;
    if (exponent == 128) {
      /* Infinity or NaN */
      f10 = F10_MAX_EXPONENT;
      if (mantissa) {
        f10 |= (mantissa & F10_MANTISSA_BITS);
      }
    } else if (exponent > 15) {
      /* Overflow - flush to Infinity */
      f10 = F10_MAX_EXPONENT;
    } else if (exponent > -15) {
      /* Representable value */
      exponent += F10_EXPONENT_BIAS;
      mantissa >>= F10_MANTISSA_SHIFT;
      f10 = exponent << F10_EXPONENT_SHIFT | mantissa;
    } else {
        f10 = 0;
    }
    return f10;
  }
}());

function rgb101111(r, g, b) {
  return (to11uf(r) <<  0) |
         (to11uf(g) << 11) |
         (to10uf(b) << 22);
}

function main() {
  const m4 = twgl.m4;
  const gl = document.querySelector("canvas").getContext("webgl2");
  if (!gl) {
    alert("your browsers/machine does not appear to support webgl2");
    return;  
  }
  
  const vs = `
     attribute vec4 position;
     attribute vec2 texcoord;

     uniform mat4 u_matrix;

     varying vec2 v_texcoord;

     void main() {
       gl_Position = u_matrix * position;
       v_texcoord = texcoord;
     }
  `;
  const fs = `
     precision mediump float;

     uniform sampler2D u_tex;

     varying vec2 v_texcoord;

     void main() {
       gl_FragColor = texture2D(u_tex, v_texcoord) * vec4(.1, 10, 1, 1);
     }
  `;
  
  const programInfo = twgl.createProgramInfo(gl, [vs, fs]);
  const bufferInfo = twgl.primitives.createCubeBufferInfo(gl, 1);
  
  const size = 8;
  const data = new Uint32Array(size * size);
  for (let y = 0; y < size; ++y) {
    for (let x = 0; x < size; ++x) {
      const u = x / (size - 1);
      const v = y / (size - 1);
      data[y * size + x] = rgb101111(u * 10, v * .1, (x + y) % 2);
    }
  }
  
  var tex = twgl.createTexture(gl, {
    internalFormat: gl.R11F_G11F_B10F,
    format: gl.RGB,
    type: gl.UNSIGNED_INT_10F_11F_11F_REV,
    minMag: gl.NEAREST,
    src: data,
  });
  
  function render(time) {
    time *= 0.001;
    
    twgl.resizeCanvasToDisplaySize(gl.canvas);
    gl.viewport(0, 0, gl.canvas.width, gl.canvas.height);
    
    gl.enable(gl.DEPTH_TEST);
    
    gl.useProgram(programInfo.program);
    
    var aspect = gl.canvas.clientWidth / gl.canvas.clientHeight;
    var matrix = m4.perspective(Math.PI / 4, aspect, 1, 100);
    matrix = m4.translate(matrix, [0, 0, -3]);
    matrix = m4.rotateY(matrix, time);
    matrix = m4.rotateX(matrix, Math.PI / 10);
    
    twgl.setBuffersAndAttributes(gl, programInfo, bufferInfo);
    twgl.setUniforms(programInfo, {
      u_matrix: matrix,
      u_tex: tex,
    });
    twgl.drawBufferInfo(gl, bufferInfo);
    requestAnimationFrame(render);
  }
  requestAnimationFrame(render);
}

main();

body { margin: 0; }
canvas { width: 100vw; height:100vh; display: block; }

<script src="https://twgljs.org/dist/2.x/twgl-full.min.js"></script>
<canvas></canvas>