Memecahkan Persediaan Segitiga WebGL

WebGL mempunyai rekod prestasi sebagai salah satu API Javascript yang lebih kompleks. Sebagai pembangun web yang tertarik dengan semua yang interaktif, saya memutuskan untuk menyelami dokumentasi MDN WebGL. Saya telah bekerja dengan Three.js yang menguraikan banyak kesusahan WebGL tetapi saya tidak dapat mengelak daripada membuka peluang!

Matlamat penyelaman dalam ini adalah untuk melihat sama ada saya dapat memahami dokumentasi yang cukup untuk menerangkannya dalam istilah yang lebih mudah. Penafian — Saya telah bekerja dengan Three.js dan mempunyai sedikit pengetahuan dalam istilah dan corak grafik 3D. Saya pasti akan menerangkan konsep ini jika ia digunakan dalam apa jua cara.

Untuk bermula, kami akan menumpukan pada penciptaan segi tiga. Sebabnya adalah untuk mewujudkan pemahaman tentang bahagian yang diperlukan untuk menyediakan webGL.

Untuk merasai perkara yang akan diliputi, berikut ialah langkah yang akan kami lalui.

• Sediakan kanvas HTML

• Dapatkan konteks WebGL

• Kosongkan warna kanvas dan tetapkan warna baharu

• Buat tatasusunan segi tiga (x,y )koordinat

• Tambahkan bucu dan kod peneduh serpihan

• Proses dan susun kod shader

• Buat program webGL

• Buat dan ikat penimbal pada program webGL

• Gunakan program

• Pautkan maklumat GPU ke CPU

• Lukiskan segitiga

Sediakan Kanvas HTML

1. Buat folder yang dipanggil "RenderTriangle"
2. Dalam folder itu buat fail index.html dan main.js

Dalam fail index.html tambah kod berikut:

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. ialah titik masuk untuk memaparkan konteks WebGL

2. Kami menyediakan kod HTML asas dan memautkan fail main.js.

3. Dalam fail main.js kami akan mengakses id kanvas untuk memaparkan kandungan webGL.

Sediakan Kanvas HTML

Dalam fail main.js tambah kod berikut yang menyediakan kanvas HTML:

utama.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. Dapatkan kanvas HTML mengikut id dan simpannya dalam pembolehubah yang dipanggil "kanvas" (anda boleh menggunakan sebarang nama).

2. Tetapkan sifat canvas.width dan canvas.height kanvas dengan mengakses window.innerWidth dan window.innerHeight . Ini menetapkan paparan yang diberikan kepada saiz tetingkap penyemak imbas.

3. Dapatkan konteks WebGL menggunakan canvas.getContext(“webgl2”) dan simpannya dalam pembolehubah yang dipanggil “gl”.

Warna Jelas

utama.js

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

Sebelum menulis sebarang program webGL, anda perlu menetapkan warna latar belakang kanvas anda. WebGL mempunyai dua kaedah untuk mewujudkan perkara ini.

clearColor() — ialah kaedah yang menetapkan warna latar belakang tertentu. Ia dipanggil "clearColor" kerana apabila anda memaparkan WebGL ke dalam kanvas HTML, CSS menetapkan latar belakang kepada warna hitam. Apabila anda memanggil clearColor(), ia mengosongkan warna lalai dan menetapkan apa sahaja warna yang anda mahu. Kita boleh lihat ini di bawah.

Nota: clearColor() mempunyai 4 parameter (r, g, b, a)

clear() — selepas clearColor() dipanggil dan warna latar belakang yang jelas ditetapkan,
**clear()** mesti dipanggil untuk “kosongkan” atau set semula penimbal kepada nilai pratetap (penampan ialah storan sementara untuk maklumat warna dan kedalaman).

Kaedah clear() ialah salah satu kaedah lukisan yang bermaksud kaedah itulah yang sebenarnya menghasilkan warna. Tanpa memanggil kaedah clear(), kanvas tidak akan menunjukkan clearColor. Pilihan parameter clear() ialah,
ialah gl.COLOR_BUFFER_BIT, gl.DEPTH_BUFFER_BIT atau gl.STENCIL_BUFFER_BIT.

Dalam kod di bawah anda boleh menambah berbilang parameter untuk ditetapkan semula dalam senario yang berbeza.

1. gl.DEPTH_BUFFER_BIT — menunjukkan penimbal untuk maklumat kedalaman piksel

2. gl.COLOR_BUFFER_BIT — menunjukkan penimbal untuk maklumat warna piksel

Tetapkan Koordinat Segi Tiga

utama.js

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

Setelah latar belakang ditetapkan, kita boleh menetapkan koordinat yang diperlukan untuk mencipta segi tiga. Koordinat disimpan dalam tatasusunan sebagai koordinat (x, y).

Tatasusunan di bawah memegang koordinat 3 mata. Titik ini bersambung untuk membentuk segi tiga.

0.0, -1.0
0.0 , 1.0
1.0, 1.0

Menambah Vertex dan Fragment Shaders

utama.js

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
}
`;

Selepas mencipta pembolehubah untuk koordinat segi tiga, kami boleh menyediakan pelorek.

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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