参考资料：
https://juejin.im/post/5d1860a2f265da1bb27746d6
并再此基础上添加了其他滤镜：二分、四分、九分、灰度、暖色、冷色、漩涡、矩形马赛克、六边形马赛克、三角形马赛克、圆形马赛克、缩放、闪白、抖动、灵魂出窍、毛刺、幻觉、浮雕。
效果图：
IMG_0800.PNG
也能够在纹理中获取处理后的UIImage
// 获取纹理对应的 UIImage，调用前先绑定对应的帧缓存
- (UIImage *)imageFromTextureWithWidth:(int)width height:(int)height {
int size = width * height * 4;
GLubyte *buffer = malloc(size);
glReadPixels(0, 0, width, height, GL_RGBA, GL_UNSIGNED_BYTE, buffer);
CGDataProviderRef provider = CGDataProviderCreateWithData(NULL, buffer, size, NULL);
int bitsPerComponent = 8;
int bitsPerPixel = 32;
int bytesPerRow = 4 * width;
CGColorSpaceRef colorSpaceRef = CGColorSpaceCreateDeviceRGB();
CGBitmapInfo bitmapInfo = kCGBitmapByteOrderDefault;
CGColorRenderingIntent renderingIntent = kCGRenderingIntentDefault;
CGImageRef imageRef = CGImageCreate(width, height, bitsPerComponent, bitsPerPixel, bytesPerRow, colorSpaceRef, bitmapInfo, provider, NULL, NO, renderingIntent);
// 此时的 imageRef 是上下颠倒的，调用 CG 的方法重新绘制一遍，刚好翻转过来
UIGraphicsBeginImageContext(CGSizeMake(width, height));
CGContextRef context = UIGraphicsGetCurrentContext();
CGContextDrawImage(context, CGRectMake(0, 0, width, height), imageRef);
UIImage *image = UIGraphicsGetImageFromCurrentImageContext();
UIGraphicsEndImageContext();
free(buffer);
self.imageView.image = image;
return image;
}
