获取 webGL2 片段着色器上的当前像素位置

Question

我创建了一个简单的 webGL 脚本，它根据 (x,y) 像素位置应用像素颜色

我得到的：

这是我所做的：

#ifdef GL_ES
precision mediump float;
#endif

uniform float width;
uniform float height;
uniform float time;

void main() {
  vec2 u_resolution = vec2(width, height);
    vec2 st = gl_FragCoord.xy / u_resolution;
    gl_FragColor = vec4(st.x, st.y, 0.5, 1.0);
}

Codepen: Hello WebGL

我正在尝试将其转换为 webGL2，但我不知道如何获取当前像素位置。

这是我的尝试：

#version 300 es
#ifdef GL_ES
precision mediump float;
#endif

uniform float width;
uniform float height;
uniform float time;
out vec4 color;

void main() {
  vec2 u_resolution = vec2(width, height);
    vec2 st = color.xy / u_resolution;
    color = vec4(st.x, st.y, 0.5, 1.0);
}

Codepen: Hello WebGL2

如何在 webgl2 中获取当前像素位置？

Answer 1

gl_FragCoord在WebGL2

中仍然是正确的方式

var canvas = document.body.appendChild(document.createElement("canvas"));
canvas.width = window.innerWidth;
canvas.height = window.innerHeight;
var gl = canvas.getContext("webgl2");

//************** Shader sources **************
var vertexSource = `
#version 300 es
in vec2 position;
void main() {
  gl_Position = vec4(position, 0.0, 1.0);
}
`;

var fragmentSource = `
#version 300 es
#ifdef GL_ES
precision mediump float;
#endif

uniform float width;
uniform float height;
uniform float time;
out vec4 color;

void main() {
  vec2 u_resolution = vec2(width, height);
 vec2 st = gl_FragCoord.xy / u_resolution;
 color = vec4(st.x, st.y, 0.5, 1.0);
}`;

window.addEventListener("resize", onWindowResize, false);

function onWindowResize() {
  canvas.width = window.innerWidth;
  canvas.height = window.innerHeight;
  gl.viewport(0, 0, canvas.width, canvas.height);
  gl.uniform1f(widthHandle, window.innerWidth);
  gl.uniform1f(heightHandle, window.innerHeight);
}

//Compile shader and combine with source
function compileShader(shaderSource, shaderType) {
  var shader = gl.createShader(shaderType);
  gl.shaderSource(shader, shaderSource);
  gl.compileShader(shader);
  if (!gl.getShaderParameter(shader, gl.COMPILE_STATUS)) {
    throw "Shader compile failed with: " + gl.getShaderInfoLog(shader);
  }
  return shader;
}

//From https://codepen.io/jlfwong/pen/GqmroZ
//Utility to complain loudly if we fail to find the attribute/uniform
function getAttribLocation(program, name) {
  var attributeLocation = gl.getAttribLocation(program, name);
  if (attributeLocation === -1) {
    throw "Cannot find attribute " + name + ".";
  }
  return attributeLocation;
}

function getUniformLocation(program, name) {
  var attributeLocation = gl.getUniformLocation(program, name);
  if (attributeLocation === -1) {
    throw "Cannot find uniform " + name + ".";
  }
  return attributeLocation;
}

//************** Create shaders **************

//Create vertex and fragment shaders
var vertexShader = compileShader(vertexSource.trim(), gl.VERTEX_SHADER);
var fragmentShader = compileShader(fragmentSource.trim(), gl.FRAGMENT_SHADER);

//Create shader programs
var program = gl.createProgram();
gl.attachShader(program, vertexShader);
gl.attachShader(program, fragmentShader);
gl.linkProgram(program);

gl.useProgram(program);

//Set up rectangle covering entire canvas
var vertexData = new Float32Array([
  -1.0,
  1.0, // top left
  -1.0,
  -1.0, // bottom left
  1.0,
  1.0, // top right
  1.0,
  -1.0 // bottom right
]);

//Create vertex buffer
var vertexDataBuffer = gl.createBuffer();
gl.bindBuffer(gl.ARRAY_BUFFER, vertexDataBuffer);
gl.bufferData(gl.ARRAY_BUFFER, vertexData, gl.STATIC_DRAW);

// Layout of our data in the vertex buffer
var positionHandle = getAttribLocation(program, "position");

gl.enableVertexAttribArray(positionHandle);
gl.vertexAttribPointer(
  positionHandle,
  2, // position is a vec2 (2 values per component)
  gl.FLOAT, // each component is a float
  false, // don't normalize values
  2 * 4, // two 4 byte float components per vertex (32 bit float is 4 bytes)
  0 // how many bytes inside the buffer to start from
);

//Set uniform handle
var timeHandle = getUniformLocation(program, "time");
var widthHandle = getUniformLocation(program, "width");
var heightHandle = getUniformLocation(program, "height");

gl.uniform1f(widthHandle, window.innerWidth);
gl.uniform1f(heightHandle, window.innerHeight);

function draw() {
  //Send uniforms to program
  gl.uniform1f(timeHandle, performance.now());
  //Draw a triangle strip connecting vertices 0-4
  gl.drawArrays(gl.TRIANGLE_STRIP, 0, 4);
  requestAnimationFrame(draw);
}

draw();

html {
  overflow: hidden;
}
canvas {
  display: block;
}

其他一些随机提示。

这些 ifdef 是不相关的

#ifdef GL_ES
precision mediump float;
#endif

刚刚

precision mediump float;

可以。

我猜这很明显，但为什么要分开传入 width 和 height？

就
怎么样
```
uniform vec2 u_resolution;
```

没有理由打电话给performance.now。时间传递给您的 requestAnimationFrame 回调

function draw(time) {
  //Send uniforms to program
  gl.uniform1f(timeHandle, time);

  ...
  requestAnimationFrame(draw);
}

requestAnimationFrame(draw);

代码检查编译错误但不检查 link 错误

您应该检查 link 个错误
```
gl.linkProgram(program);
if (!gl.getProgramParameter(program, gl.LINK_STATUS)) {
  throw "Program link failed with: " + gl.getProgramInfoLog(program);
}
```
如果您的变量不匹配，将会出现 link 错误，而且规范不需要编译即使在糟糕的着色器上也不会失败。相反，它只需要如果它们不好就不能 link.
window.innerWidth

参见：this
gl.getUniformLocation returns null 如果制服不存在

代码正在检查 -1，它对属性正确但对制服不正确。
抛出不存在的属性和制服

当然知道它们不存在是有帮助的，但通过注释或编辑来调试着色器是很常见的。例如，假设屏幕上没有任何内容。如果是我，我要做的第一件事就是将片段着色器更改为此
```
const fragmentSource = `
#version 300 es
precision mediump float;

uniform vec2 u_resolution;
uniform float time;
out vec4 color;

void main() {
    vec2 st = gl_FragCoord.xy / u_resolution;
    color = vec4(st.x, st.y, 0.5, 1.0);
    color = vec4(1, 0, 0, 1);  // <----------------------
}`;
```
只需输出纯色即可检查问题出在片段着色器还是顶点着色器中。在我这样做的那一刻，大多数 WebGL 实现将优化 u_resolution 并且在有效查找位置时抛出的代码使程序无法调试。

事实上，代码目前只运行，因为之前的错误检查是 -1 而不是 null。修复了该错误后，代码崩溃了，因为 time 被优化了。

var canvas = document.body.appendChild(document.createElement("canvas"));
var gl = canvas.getContext("webgl2");

//************** Shader sources **************
var vertexSource = `
#version 300 es
in vec2 position;
void main() {
  gl_Position = vec4(position, 0.0, 1.0);
}
`;

var fragmentSource = `
#version 300 es
precision mediump float;

uniform vec2 u_resolution;
uniform float time;
out vec4 color;

void main() {
  vec2 st = gl_FragCoord.xy / u_resolution;
  color = vec4(st.x, st.y, 0.5, 1.0);
}`;


function resize() {
  if (canvas.width !== canvas.clientWidth || canvas.height !== canvas.clientHeight) {
    canvas.width = canvas.clientWidth;
    canvas.height = canvas.clientHeight;
    gl.viewport(0, 0, canvas.width, canvas.height);
    gl.uniform2f(resHandle, canvas.width, canvas.height);
  }
}

//Compile shader and combine with source
function compileShader(shaderSource, shaderType) {
  var shader = gl.createShader(shaderType);
  gl.shaderSource(shader, shaderSource);
  gl.compileShader(shader);
  if (!gl.getShaderParameter(shader, gl.COMPILE_STATUS)) {
    throw "Shader compile failed with: " + gl.getShaderInfoLog(shader);
  }
  return shader;
}

//From https://codepen.io/jlfwong/pen/GqmroZ
//Utility to complain loudly if we fail to find the attribute/uniform
function getAttribLocation(program, name) {
  var attributeLocation = gl.getAttribLocation(program, name);
  if (attributeLocation === -1) {
    console.warn("Cannot find attribute", name);
  }
  return attributeLocation;
}

function getUniformLocation(program, name) {
  var uniformLocation = gl.getUniformLocation(program, name);
  if (uniformLocation === null) {
    console.warn("Cannot find uniform", name);
  }
  return uniformLocation;
}

//************** Create shaders **************

//Create vertex and fragment shaders
var vertexShader = compileShader(vertexSource.trim(), gl.VERTEX_SHADER);
var fragmentShader = compileShader(fragmentSource.trim(), gl.FRAGMENT_SHADER);

//Create shader programs
var program = gl.createProgram();
gl.attachShader(program, vertexShader);
gl.attachShader(program, fragmentShader);
gl.linkProgram(program);
if (!gl.getProgramParameter(program, gl.LINK_STATUS)) {
  throw "Program link failed with: " + gl.getProgramInfoLog(program);
}

gl.useProgram(program);

//Set up rectangle covering entire canvas
var vertexData = new Float32Array([
  -1.0,
  1.0, // top left
  -1.0,
  -1.0, // bottom left
  1.0,
  1.0, // top right
  1.0,
  -1.0 // bottom right
]);

//Create vertex buffer
var vertexDataBuffer = gl.createBuffer();
gl.bindBuffer(gl.ARRAY_BUFFER, vertexDataBuffer);
gl.bufferData(gl.ARRAY_BUFFER, vertexData, gl.STATIC_DRAW);

// Layout of our data in the vertex buffer
var positionHandle = getAttribLocation(program, "position");

gl.enableVertexAttribArray(positionHandle);
gl.vertexAttribPointer(
  positionHandle,
  2, // position is a vec2 (2 values per component)
  gl.FLOAT, // each component is a float
  false, // don't normalize values
  2 * 4, // two 4 byte float components per vertex (32 bit float is 4 bytes)
  0 // how many bytes inside the buffer to start from
);

//Set uniform handle
var timeHandle = getUniformLocation(program, "time");
var resHandle = getUniformLocation(program, "u_resolution");

function draw(time) {
  resize();
  //Send uniforms to program
  gl.uniform1f(timeHandle, time);
  //Draw a triangle strip connecting vertices 0-4
  gl.drawArrays(gl.TRIANGLE_STRIP, 0, 4);
  requestAnimationFrame(draw);
}

requestAnimationFrame(draw);

html,body {
  height: 100%;
  margin: 0;
}
canvas {
  width: 100%;
  height: 100%;
  display: block;
}

获取 webGL2 片段着色器上的当前像素位置

get current pixel position on webGL2 fragment shader

webgl

webgl2