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];}