如何使用php中的imagejpeg在Kb中设置特定压缩

Question

I'm trying to compress uploaded images down to a specific size of 200Kb. I don't want to compress them any more than they need to be and using lossless compression like PNG isn't enough. Simply setting it to imagejpeg($image, null, 40) creates different compressed sizes for different images. Is there a way to set the desired compression size in bytes or at least have some algorithm that can find out the compression output without looping through imagejpeg() from 100 to 0 quality?

Answer 1

There really isn't any way to predict the level of compression beforehand. The effect of compression depends upon the image source. One of the problems is that there is a myriad of JPEG compression settings.

1. The quantization tables (up to 3) with 64 different values.
2. Sub sampling.
3. Spectral selection in progressive scans.
4. Successive Approximation in progressive scans.
5. Optimize huffman tables.

So there are billions upon billions of parameters that you can set.

There are JPEG optimization applications out there that will look at the compressed data.