问题描述
JPEG是一种有损压缩方案，因此解压缩操作重新压缩通常会进一步降低每个步骤的图像质量。 是否可以旋转JPEG图片，而不会导致进一步的损失？从我知道的JPEG算法，我天真似乎可以避免进一步的损失与一点努力。
JPEG is a lossy compression scheme, so decompression-manipulation-recompression normally reduces the image quality further for each step. Is it possible to rotate a JPEG image without incurring further loss? From what little I know of the JPEG algorithm, it naively seems possible to avoid further loss with a bit of effort. Which common image manipulation programs (e.g. GIMP, Paint Shop Pro, Windows Photo Gallery) and graphic libraries cause quality loss when performing a rotation and which don't?
推荐答案
是的，在某些情况下是可能的：在尺寸为8的倍数的图像上进行90度旋转和翻转.JPEG算法的核心 - 有损部分 - 涉及破坏图像转换为8x8像素块，对块执行，然后量化结果。还有一些颜色空间转换和在这之上的块的无损压缩。
Yes, it is possible for certain cases: 90-degree rotations and flips on images with dimensions that are a multiple of 8. The heart of the JPEG algorithm -- the lossy part -- involves breaking the image into 8x8 pixel blocks, performing a discrete cosine transform on the block and then quantizing the result. There's also some color space conversion and lossless compression of the blocks on top of this.
旋转或翻转8x8块将给予具有相同基本系数的DCT，但可能转置和/或根据转换具有一些符号变化。因此，无损地旋转或翻转图片的基本步骤包括：
Rotating or flipping an 8x8 block will give a DCT with the same basic coefficients, but possibly transposed and/or with some sign changes depending on the transformation. So the basic steps to rotate or flip an image losslessly would involve:
- 解压缩并提取块
- 将每个块的DCT系数转置和/或符号翻转
- 将块重新整理为新顺序（否则8x8块将旋转，但仍在旧位置） >
- 使用无损压缩步骤重新压缩所有文件。
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