分解 WebGL 三角形設置

WebGL 一直是 Javascript 中更複雜的 API 之一。作為一名對所有互動式內容感興趣的 Web 開發人員，我決定深入研究 WebGL 的 MDN 文件。我曾經使用過 Three.js，它抽象化了 WebGL 的許多困難，但我還是忍不住打開了它！

這次深入研究的目的是看看我是否能夠充分理解文件並用更簡單的術語進行解釋。 免責聲明 — 我曾使用過 Three.js，並且對 3D 圖形術語和模式有一定的了解。如果這些概念以任何方式適用，我一定會解釋它們。

首先，我們將重點放在三角形的創建。這樣做的原因是為了了解設定 webGL 所需的部分。

為了了解將要涵蓋的內容，以下是我們將要執行的步驟。

• 設定 HTML 畫布

• 取得 WebGL 上下文

• 清除畫布顏色並設定新顏色

• 建立三角形 (x,y ) 座標數組

• 新增頂點和片段著色器程式碼

• 處理並編譯著色器程式碼

• 建立 webGL 程式

• 建立緩衝區並將其綁定到 webGL 程式

• 使用該程式

• 將GPU訊息連結到CPU

• 畫出三角形

設定 HTML 畫布

1. 建立一個名為「RenderTriangle」的資料夾
2. 在該資料夾中建立一個index.html和main.js檔案

在index.html檔案中加入以下程式碼：

index.js

<meta charset="UTF-8">
<meta name="viewport" content="width=device-width, initial-scale=1.0">
<title>Document</title>


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

<script src="main.js"></script>

1. 是渲染 WebGL 上下文的入口點

2. 我們設定了基本的 HTML 程式碼並連結 main.js 檔案。

3. 在 main.js 檔案中，我們將存取畫布 id 來渲染 webGL 內容。

準備 HTML 畫布

在 main.js 檔案中加入以下程式碼來準備 HTML 畫布：

main.js

// get canvas
const canvas = document.getElementById("canvas");

// set width and height of canvas
canvas.width = window.innerWidth;

canvas.height = window.innerHeight;

// get the webgl context
const gl = canvas.getContext("webgl2");

1. 透過id取得HTML畫布並將其儲存在名為「canvas」的變數中（您可以使用任何名稱）。

2. 透過存取window.innerWidth 和window.innerHeight 設定畫布的canvas.width 和canvas.height

使用
3. canvas.getContext(“webgl2”)

   取得 WebGL 上下文並將其儲存在名為「gl」的變數中。

色彩清晰

main.js

在編寫任何 webGL 程式之前，您需要設定畫布的背景顏色。 WebGL 有兩種方法可以實現這一點。

gl.clearColor(0.1, 0.2, 0.3, 1.0);
gl.clear(gl.DEPTH_BUFFER_BIT | gl.COLOR_BUFFER_BIT);

clearColor()

— 是一種設定特定背景顏色的方法。之所以稱為“clearColor”，是因為當您將 WebGL 渲染到 HTML 畫布中時，CSS 會將背景設為黑色。當您呼叫 clearColor() 時，它會清除預設顏色並設定您想要的任何顏色。我們可以在下面看到這一點。

注意：

clearColor()

有 4 個參數 (r, g, b, a)

clear()

— 在呼叫 clearColor() 並設定顯式背景顏色後， **clear()** 必須呼叫來「清除」或將緩衝區重設為預設值（緩衝區是顏色和深度資訊的暫存）。

clear()

方法是繪圖方法之一，這表示它是實際渲染顏色的方法。如果不呼叫 clear() 方法，畫布將不會顯示 clearColor。 clear() 參數選項是， 是 gl.COLOR_BUFFER_BIT、gl.DEPTH_BUFFER_BIT 或 gl.STENCIL_BUFFER_BIT。
在下面的程式碼中您可以新增多個參數以在不同場景下重置。

gl.DEPTH_BUFFER_BIT — 表示像素深度資訊的緩衝區
gl.COLOR_BUFFER_BIT — 表示像素顏色資訊的緩衝區
設定三角形座標

main.js

設定背景後，我們可以設定建立三角形所需的座標。座標以 (x, y) 座標的形式儲存在數組中。

// set position coordinates for shape
const triangleCoords = [0.0, -1.0, 0.0, 1.0, 1.0, 1.0];

下面的陣列保存 3 點座標。這些點連接起來形成三角形。

0.0, -1.0

0.0 , 1.0

1.0, 1.0

新增頂點和片段著色器

main.js

為三角形座標建立變數後，我們可以設定著色器。

const vertexShader = `#version 300 es
precision mediump float;

in vec2 position;

void main() {
 gl_Position = vec4(position.x, position.y, 0.0, 1.0); //x,y,z,w
}
`;


const fragmentShader = `#version 300 es
precision mediump float;

out vec4 color;

void main () {
 color = vec4(0.0,0.0,1.0,1.0); //r,g,b,a
}
`;

A shader is a program written in OpenGL ES Shading Language. The program takes position and color information about each vertice point. This information is what’s needed to render geometry.

There are two types of shaders functions that are needed to draw webgl content, the vertex shader and fragment shader

*vertex shader *— The vertex shader function uses position information to render each pixel. Per every render, the vertex shader function runs on each vertex. The vertex shader then transforms each vertex from it’s original coordinates to WebGL coordinates. Each transformed vertex is then saved to the gl_Position variable in the vertex shader program.

*fragment shader *— The fragment shader function is called once for every pixel on a shape to be drawn. This occurs after the vertex shader runs. The fragment shader determines how the color of each pixel and “texel (pixel within a texture)” should be applied. The fragment shader color is saved in the gl_FragColor variable in the fragment shader program.

In the code above we are creating both a vertexShader and fragmentShader constant and storing the shader code in them. The Book of Shaders is a great resource for learning how to write GLSL code.

Processing the Vertex and Fragment Shaders

main.js

// process vertex shader
const shader = gl.createShader(gl.VERTEX_SHADER);

gl.shaderSource(shader, vertexShader);

gl.compileShader(shader);

if (!gl.getShaderParameter(shader, gl.COMPILE_STATUS)) {
console.log(gl.getShaderInfoLog(vertexShader));
}


// process fragment shader
const shader = gl.createShader(gl.FRAGMENT_SHADER);

gl.shaderSource(shader, fragmentShader);

gl.compileShader(shader);

if (!gl.getShaderParameter(shader, gl.COMPILE_STATUS)) {
console.log(gl.getShaderInfoLog(fragmentShader));
}

Now that we wrote the shader code (GLSL), we need to create the shader. The shader code still needs to compile. To do this we call the following functions:

createShader() — This creates the shader within the WebGL context

shaderSource() — This takes the GLSL source code that we wrote and sets it into the webGLShader object that was created with createShader.

compileShader() — This compiles the GLSL shader program into data for the WebGLProgram.

The code above processes the vertex and fragment shaders to eventually compile into the WebGLProgram.

Note: An if conditional is added to check if both shaders have compiled properly. If not, an info log will appear. Debugging can be tricky in WebGL so adding these checks is a must.

Creating a WebGL Program

main.js

const program = gl.createProgram();

// Attach pre-existing shaders
gl.attachShader(program, vertexShader);
gl.attachShader(program, fragmentShader);

gl.linkProgram(program);

if (!gl.getProgramParameter(program, gl.LINK_STATUS)) {
  const info = gl.getProgramInfoLog(program);
  throw "Could not compile WebGL program. \n\n${info}";
}

Let’s review the code above:

After compiling the vertexShader and fragmentShader, we can now create a WebGLProgram. A WebGLProgram is an object that holds the compiled vertexShader and fragmentShader.

createProgram() — Creates and initializes the WebGLProgram

attachShader() — This method attaches the shader to the webGLProgram

linkProgram() — This method links the program object with the shader objects

Lastly, we need to make a conditional check to see if the program is running properly. We do this with the gl.getProgramParameter.

Create and Bind the Buffers

Now that the WebGL Program is created and the shader programs are linked to it, it’s time to create the buffers. What are buffers?

To simplify it as much as possible, buffers are objects that store vertices and colors. Buffers don’t have any methods or properties that are accessible. Instead, the WebGL context has it’s own methods for handling buffers.

Buffer — “a temporary storage location for data that’s being moved from one place to another”
-wikipedia

We need to create a buffer so that we can store our triangle colors and vertices.

To do this we add the following:

main.js

const buffer = gl.createBuffer();
gl.bindBuffer(gl.ARRAY_BUFFER, buffer);

gl.bufferData(gl.ARRAY_BUFFER, new Float32Array(triangleCoords), gl.STATIC_DRAW);

gl.bindBuffer(gl.ARRAY_BUFFER, null);

In the code above we’re creating a buffer (or temporary storage object) using the createBuffer() method. Then we store it in a constant. Now that we have a buffer object we need to bind it to a target. There are several different target but since we’re storing coordinates in an array we will be using the gl.ARRAY_BUFFER.

To bind the buffer to a target, we use gl.bindBuffer() and pass in the gl.ARRAY_BUFFER (target) and the buffer itself as parameters.

The next step would be to use gl.bufferData() which creates the data store. gl.bufferData() takes the following parameters:

target — gl.ARRAY_BUFFER
data — new Float32Array(triangleCoords)
usage (draw type) — gl.STATIC_DRAW

Lastly, we unbind the buffer from the target to reduce side effects.

Use Program

main.js

gl.useProgram(program);

Once the buffer creation and binding is complete, we can now call the method that sets the WebGLProgram to the rendering state.

Link GPU and CPU

As we get closer to the final step, we need to talk about attributes.

In WebGL, vertex data is stored in a special variable called attributes. attributes are only available to the javascript program and the vertex shader. To access the attributes variable we need to first get the location of the attributes from the GPU. The GPU uses an index to reference the location of the attributes.

main.js

// get index that holds the triangle position information
const position = gl.getAttribLocation(obj.program, obj.gpuVariable);

gl.enableVertexAttribArray(position);

gl.bindBuffer(gl.ARRAY_BUFFER, buffer);

gl.vertexAttribPointer(position, 2, gl.FLOAT, obj.normalize, obj.stride, obj.offset);

Let’s review the code above:

Since we’re rendering a triangle we need the index of the position variable that we set in the vertex shader. We do this using the gl.getAttribLocation() method. By passing in the WebGL program and the position variable name (from the vertex shader) we can get the position attribute’s index.

Next, we need to use the gl.enableVertexAttribArray() method and pass in the position index that we just obtained. This will enable the attributes` storage so we can access it.

We will then rebind our buffer using gl.bindBuffer() and pass in the gl.ARRAY_BUFFER and buffer parameters (the same as when we created the buffers before). Remember in the “Create and Bind the Buffers” section we set the buffer to null to avoid side effects.

When we binded the buffer to the gl.ARRAY_BUFFER we are now able to store our attributes in a specific order. gl.vertexAttribPointer() allows us to do that.

By using gl.vertexAttribPointer() we can pass in the attributes we’d like to store in a specific order. The parameters are ordered first to last.

The gl.vertexAttribPointer is a more complex concept that may take some additional research. You can think of it as a method that allows you to store your attributes in the vertex buffer object in a specific order of your choosing.

Sometimes 3D geometry already has a certain format in which the geometry information is set. vertexAttribPointer comes in handy if you need to make modifications to how that geometry information is organized.

Draw Triangle

main.js

gl.drawArrays(gl.TRIANGLES, 0, 3);

Lastly, we can use the gl.drawArrays method to render the triangle. There are other draw methods, but since our vertices are in an array, this method should be used.

gl.drawArrays() takes three parameters:

mode — which specifies the type of primitive to render. (in this case were rendering a triangle)

first — specifies the starting index in the array of vector points (our triangle coordinates). In this case it’s 0.

count — specifies the number of indices to be rendered. ( since it's a triangle we’re rendering 3 indices)

Note: For more complex geometry with a lot of vertices you can use **triangleCoords.length / 2 **to ****get how many indices your geometry has.

Finally, your triangle should be rendered to the screen! Let’s review the steps.

• Set up the HTML canvas

• Get the WebGL context

• Clear the canvas color and set a new one

• Create an array of triangle (x,y )coordinates

• Add vertex and fragment shader code

• Process and compile the shader code

• Create a webGL program

• Create and bind buffers to the webGL program

• Use the program

• Link the GPU information to the CPU

• Draw out the triangle

The API is a complex one so there’s still a lot to learn but understanding this setup has given me a better foundation.

// Set up the HTML canvas
const canvas = document.getElementById("canvas");

canvas.width = window.innerWidth;
canvas.height = window.innerHeight;

// get the webgl context
const gl = canvas.getContext("webgl2");

// Clear the canvas color and set a new one
gl.clearColor(0.1, 0.2, 0.3, 1.0);
gl.clear(gl.DEPTH_BUFFER_BIT | gl.COLOR_BUFFER_BIT);


// Create an array of triangle (x,y )coordinates
const triangleCoords = [0.0, -1.0, 0.0, 1.0, 1.0, 1.0];

// Add vertex and fragment shader code
const vertexShader = `#version 300 es
precision mediump float;
in vec2 position;

void main() {
gl_Position = vec4(position.x, position.y, 0.0, 1.0); //x,y,z,w
}
`;

const fragmentShader = `#version 300 es
precision mediump float;
out vec4 color;

void main () {
color = vec4(0.0,0.0,1.0,1.0); //r,g,b,a
}
`;


// Process and compile the shader code
const vShader = gl.createShader(gl.VERTEX_SHADER);

gl.shaderSource(vShader, vertexShader);
gl.compileShader(vShader);

if (!gl.getShaderParameter(vShader, gl.COMPILE_STATUS)) {
 console.log(gl.getShaderInfoLog(vertexShader));
}


const fShader = gl.createShader(gl.FRAGMENT_SHADER);

gl.shaderSource(fShader, fragmentShader);
gl.compileShader(fShader);

if (!gl.getShaderParameter(fShader, gl.COMPILE_STATUS)) {
console.log(gl.getShaderInfoLog(fragmentShader));
}

// Create a webGL program
const program = gl.createProgram();


// Link the GPU information to the CPU
gl.attachShader(program, vShader);
gl.attachShader(program, fShader);

gl.linkProgram(program);


if (!gl.getProgramParameter(program, gl.LINK_STATUS)) {
const info = gl.getProgramInfoLog(program);
throw "Could not compile WebGL program. \n\n${info}";
}


// Create and bind buffers to the webGL program
const buffer = gl.createBuffer();

gl.bindBuffer(gl.ARRAY_BUFFER, buffer);
gl.bufferData(gl.ARRAY_BUFFER, new Float32Array(triangleCoords), gl.STATIC_DRAW);
gl.bindBuffer(gl.ARRAY_BUFFER, null);


// Use the program
gl.useProgram(program);



// Link the GPU information to the CPU
const position = gl.getAttribLocation(program, "position");

gl.enableVertexAttribArray(position);
gl.bindBuffer(gl.ARRAY_BUFFER, buffer);
gl.vertexAttribPointer(position, 2, gl.FLOAT, gl.FALSE, 0, 0);


// render triangle
gl.drawArrays(gl.TRIANGLES, 0, 3);

Here are some invaluable references to help understand this material better.

https://developer.mozilla.org/en-US/docs/Web/API/WebGL_API
https://www.udemy.com/course/webgl-internals/
https://webglfundamentals.org/

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