HT for Web基于HTML5的图像操作（三）

浏览数：38 / 时间：2015年06月09日

上篇采用了HTML5的Canvas的globalCompositeOperation属性达到了染色效果，其实CSS也提供了一些常规图像变化的设置参数，关于CSS的过滤器Filter设置可参考 http://www.html5rocks.com/en/tutorials/filters/understanding-css/，CSS的设置的确很方便，不需要我们了解底层实现逻辑，仅需要提供简单的设置参数值即可，但这样的方式毕竟无法达到随心所欲变化的效果，这有点想OpenGL早期只能通过Fixed Function Pipeline进行开发，和如今采用Shading Language可实现丰富多彩变化的质变，因此这篇我们将采用WebGL的SL代码方式折腾点更好玩的效果。

在第一篇我们已经提到图像操作的本质是对输入输出的数据变化，因此染色仅是最简单直观的基本变化，通过WebGL的SL代码我们甚至可以实现图像扭曲等变化效果，先上张最终变化效果的截图，原图就是Hightopo官网的banner，染色+扭曲的效果如上:

Vertex代码如下

attribute vec2 aVertexPosition;
attribute vec2 aTexturePosition;
varying vec2 vTexturePosition;
void main() {
    vTexturePosition = aTexturePosition;
    gl_Position = vec4(aVertexPosition, 0.0, 1.0); 
}

Fragment代码如下：

precision mediump float;

varying vec2 vTexturePosition;            
uniform sampler2D uSampler; 

uniform vec4 uColor; 
uniform float uRadius;
uniform float uAngle;
uniform vec2 uCenter;            

void main() {                   
   vec2 vec = vTexturePosition - uCenter;
   float distance = length(vec);
   if (distance < uRadius) {
       float ratio = (uRadius - distance) / uRadius;
       float angle = ratio * ratio * uAngle;
       float s = sin(angle);
       float c = cos(angle);
       vec = vec2(vec.x * c - vec.y * s, vec.x * s + vec.y * c);
   }                               
   gl_FragColor = texture2D(uSampler, uCenter+vec) * uColor; 
}

相应JavaScript代码如下：

    gl.viewport(0, 0, gl.viewportWidth, gl.viewportHeight);
    gl.clearColor(0.0, 0.0, 0.0, 1.0);
    gl.clear(gl.COLOR_BUFFER_BIT);

    vertexShader = loadShaderFromDOM("shader-vs");
    fragmentShader = loadShaderFromDOM("shader-fs");

    var program = gl.createProgram();
    gl.attachShader(program, vertexShader);
    gl.attachShader(program, fragmentShader);
    gl.linkProgram(program);

    gl.useProgram(program);

    var vertexBuffer = gl.createBuffer();
    gl.bindBuffer(gl.ARRAY_BUFFER, vertexBuffer);
    gl.bufferData(gl.ARRAY_BUFFER, new Float32Array([
        -0.7, 0.7,
        -0.7, -0.7,
        0.7, 0.7,
        -0.7, -0.7,
        0.7, -0.7,
        0.7, 0.7
    ]), gl.STATIC_DRAW);
    var vertexLocation = gl.getAttribLocation(program, "aVertexPosition");
    gl.vertexAttribPointer(vertexLocation, 2, gl.FLOAT, false, 0, 0);
    gl.enableVertexAttribArray(vertexLocation);

    var textureBuffer = gl.createBuffer();
    gl.bindBuffer(gl.ARRAY_BUFFER, textureBuffer);
    gl.bufferData(gl.ARRAY_BUFFER, new Float32Array([
        0, 1,
        0, 0,
        1, 1,
        0, 0,
        1, 0,
        1, 1
    ]), gl.STATIC_DRAW);
    var textureLocation = gl.getAttribLocation(program, "aTexturePosition");
    gl.vertexAttribPointer(textureLocation, 2, gl.FLOAT, false, 0, 0);
    gl.enableVertexAttribArray(textureLocation);

    gl.activeTexture(gl.TEXTURE0);
    gl.bindTexture(gl.TEXTURE_2D, texture);
    gl.uniform1i(gl.getUniformLocation(program, ‘uSampler‘), 0);                
    gl.uniform4fv(gl.getUniformLocation(program, ‘uColor‘), toRGBA(formPane.v(‘color‘)));
    gl.uniform1f(gl.getUniformLocation(program, ‘uRadius‘), formPane.v(‘radius‘));
    gl.uniform1f(gl.getUniformLocation(program, ‘uAngle‘), formPane.v(‘angle‘));
    gl.uniform2fv(gl.getUniformLocation(program, ‘uCenter‘), [formPane.v(‘centerX‘), 1-formPane.v(‘centerY‘)]);            
    gl.drawArrays(gl.TRIANGLES, 0, 6);
}