iOS实现图像素描效果

　　使用GPUImageSketchFilter对象实现图像素描效果

NSString *const kGPUImageSketchFragmentShaderString = SHADER_STRING

(

 precision mediump float;

 varying vec2 textureCoordinate;

 varying vec2 leftTextureCoordinate;

 varying vec2 rightTextureCoordinate;

 varying vec2 topTextureCoordinate;

 varying vec2 topLeftTextureCoordinate;

 varying vec2 topRightTextureCoordinate;

 varying vec2 bottomTextureCoordinate;

 varying vec2 bottomLeftTextureCoordinate;

 varying vec2 bottomRightTextureCoordinate;

 uniform float edgeStrength;

 uniform sampler2D inputImageTexture;

 void main()

 {

     float bottomLeftIntensity = texture2D(inputImageTexture, bottomLeftTextureCoordinate).r;

     float topRightIntensity = texture2D(inputImageTexture, topRightTextureCoordinate).r;

     float topLeftIntensity = texture2D(inputImageTexture, topLeftTextureCoordinate).r;

     float bottomRightIntensity = texture2D(inputImageTexture, bottomRightTextureCoordinate).r;

     float leftIntensity = texture2D(inputImageTexture, leftTextureCoordinate).r;

     float rightIntensity = texture2D(inputImageTexture, rightTextureCoordinate).r;

     float bottomIntensity = texture2D(inputImageTexture, bottomTextureCoordinate).r;

     float topIntensity = texture2D(inputImageTexture, topTextureCoordinate).r;

     float h = -topLeftIntensity - 2.0 * topIntensity - topRightIntensity + bottomLeftIntensity + 2.0 * bottomIntensity + bottomRightIntensity;

     float v = -bottomLeftIntensity - 2.0 * leftIntensity - topLeftIntensity + bottomRightIntensity + 2.0 * rightIntensity + topRightIntensity;

     float mag = 1.0 - (length(vec2(h, v)) * edgeStrength);

     gl_FragColor = vec4(vec3(mag), 1.0);

 }

);

+ (UIImage *)applySketchFilter:(UIImage *)image

{

    GPUImageSketchFilter *filter = [[GPUImageSketchFilter alloc] init];

    [filter forceProcessingAtSize:image.size];

    GPUImagePicture *pic = [[GPUImagePicture alloc] initWithImage:image];

    [pic addTarget:filter];

    [pic processImage];

    [filter useNextFrameForImageCapture];

    return [filter imageFromCurrentFramebuffer];

}