# WebGL繪製三角形

## 繪製三角形所需的步驟

```var vertices = [
-0.5,0.5,0.0,
-0.5,-0.5,0.0,
0.5,-0.5,0.0,
];

indices = [0,1,2]; ```

• 頂點著色器 - 在程式的頂點著色器，我們定義向量屬性來儲存三維坐標，並指定為 gl_position。
```var vertCode =
'attribute vec3 coordinates;' +

'void main(void) {' +
' gl_Position = vec4(coordinates, 1.0);' +
'}';```
• 片段著色器 - 在片段著色器，我們只分配碎片顏色到gl_FragColor變數。
```var fragCode = 'void main(void) {' +
' gl_FragColor = vec4(1, 0.5, 0.0, 1);' +
'}';```

`gl.drawElements(gl.TRIANGLES, indices.length, gl.UNSIGNED_SHORT,0);`

## 範例 - 繪製一個三角形

```<!doctype html>
<html>
<body>
<canvas width = "300" height = "300" id = "my_Canvas"></canvas>

<script>

/*============== Creating a canvas ====================*/
var canvas = document.getElementById('my_Canvas');
gl = canvas.getContext('experimental-webgl');

/*======== Defining and storing the geometry ===========*/

var vertices = [
-0.5,0.5,0.0,
-0.5,-0.5,0.0,
0.5,-0.5,0.0,
];

indices = [0,1,2];

// Create an empty buffer object to store vertex buffer
var vertex_buffer = gl.createBuffer();

// Bind appropriate array buffer to it
gl.bindBuffer(gl.ARRAY_BUFFER, vertex_buffer);

// Pass the vertex data to the buffer
gl.bufferData(gl.ARRAY_BUFFER, new Float32Array(vertices), gl.STATIC_DRAW);

// Unbind the buffer
gl.bindBuffer(gl.ARRAY_BUFFER, null);

// Create an empty buffer object to store Index buffer
var Index_Buffer = gl.createBuffer();

// Bind appropriate array buffer to it
gl.bindBuffer(gl.ELEMENT_ARRAY_BUFFER, Index_Buffer);

// Pass the vertex data to the buffer
gl.bufferData(gl.ELEMENT_ARRAY_BUFFER, new Uint16Array(indices), gl.STATIC_DRAW);

// Unbind the buffer
gl.bindBuffer(gl.ELEMENT_ARRAY_BUFFER, null);

var vertCode =
'attribute vec3 coordinates;' +

'void main(void) {' +
' gl_Position = vec4(coordinates, 1.0);' +
'}';

// Create a vertex shader object

// Attach vertex shader source code

var fragCode =
'void main(void) {' +
' gl_FragColor = vec4(0.0, 0.0, 0.0, 0.1);' +
'}';

// Attach fragment shader source code

// Create a shader program object to store

// Use the combined shader program object

/*======= Associating shaders to buffer objects =======*/

// Bind vertex buffer object
gl.bindBuffer(gl.ARRAY_BUFFER, vertex_buffer);

// Bind index buffer object
gl.bindBuffer(gl.ELEMENT_ARRAY_BUFFER, Index_Buffer);

// Get the attribute location

// Tutorials an attribute to the currently bound VBO
gl.vertexAttribTutorialser(coord, 3, gl.FLOAT, false, 0, 0);

// Enable the attribute
gl.enableVertexAttribArray(coord);

/*=========Drawing the triangle===========*/

// Clear the canvas
gl.clearColor(0.5, 0.5, 0.5, 0.9);

// Enable the depth test
gl.enable(gl.DEPTH_TEST);

// Clear the color buffer bit
gl.clear(gl.COLOR_BUFFER_BIT);

// Set the view port
gl.viewport(0,0,canvas.width,canvas.height);

// Draw the triangle
gl.drawElements(gl.TRIANGLES, indices.length, gl.UNSIGNED_SHORT,0);

</script>

</body>
</html>```