为了获得最佳效果，您是否应该重新调整图像大小然后对其进行压缩，还是应该对其进行压缩然后重新调整大小？

Question

I'm trying to cut down on a picture size and I was wondering:

Is it better to compress the original image first, then re-size it or should I re-size it and then compress the smaller version of it?

Is there any difference?

I'm working with C#.

Answer 1

压缩会导致图像质量和细节丢失，因此一般来说，应用后应始终是应用任何修改后的最后一步。

Answer 2

No matter what you do, compression always happens last. This is because compression is inherent in the saving of the image, and it is impractical to try to work with the image in its compressed form; you virtually always uncompress it in order to work with the image itself, and not simply pass the file around.

So, if you compress, then resize, the sequence of steps is compress->uncompress->resize->compress. If you are using a "lossless" compressed image format, like PNG, you're wasting time on the initial two steps because nothing will change, and you might as well just resize, then compress.

Where this might have a difference is when using "lossy" compression algorithms like JPEG. "Lossy" formats use similar algorithms to actually compress the data of the image as lossless ones, but they incorporate an additional step of simplification; the image is processed to produce (hopefully) subtle changes in the actual pixels, which allows the entire image to be digested more efficiently by the compression algorithm (usually, blocks of pixels are made more similar, so that the entire block can be represented by one or two bytes of "hash" which is then cross-referenced in a metadata dictionary and the hash substituted for the block where it occurs). This simplification is one-way; you can't "un-simplify" the processed image to get the original one back, and so the sequence of steps for a compress and then resize is simplify->compress->uncompress->resize->simplify->compress. The simplification step happens twice, affecting the image before and after resizing. Whether this actually results in a smaller file size depends on the exact image and the quality settings used to apply the simplification filter.

Answer 3

if you resize image before compress it is better for quality of image but bad for streaming because or data size

if you compress and resize it is better for streaming (small size) but quality will be limited at original size