The first is the typical progression from file size to set quality level of a JPEG: In order to understand how the tools determine the quality factor necessary for recompression, we should first look at the following two diagrams, which I have generated from the sample image. Adobe Photoshop: 1-12), here is a hint that I will use the system which is most common and also used by mozJPEG: The quality scale from 1-100. Since in each tool the quantization tables are set using different scaled quality controls (e. Much more interesting, however, is the quality level (actually adjusting the values for the quantization tables, see Finally understanding JPG).
Mozilla Research's mozJPEG is currently managed by Kornel, who has already made an excellent name for himself in the field of PNG compression with pngquant. I assume that most of the tools, like me, have chosen MozJPEG, which is currently the most mature of them all. The available JPEG encoders are now quite exhausted. Two things are decisive for the new compression: Firstly, the JPEG encoder used for adjustable features such as color subsampling, and secondly, the selected quality level. removal of meta data, baseline to progressive reconstruction, optimization of the Huffman tables) will normally not get much out of an image. Normally, your images will be compressed again, because measures that do not change the image data (e. How do the compression tools manage this? The tools seem to work a little magic and give you back a JPEG, which looks identical to the original with a much smaller file size. I will use this algorithm to compare the results of the tools by comparing the produced JPEG with the original image.įor better readability and comprehensibility, I multiply the resulting value by 1000 and subtract it from 1000 so that I effectively get a value for the visual distance of two images.īased on my very individual perception, I created the following evaluation table for the JPEG format: SSIM value
It tries to generate an absolute value that represents the perceived difference between two images. Some clever minds have thought about it and developed the SSIM algorithm to simulate the perception of our eyes. How do you compare the quality of two images?Īt which quality level do they look good enough?Īnd as of when do compression artifacts interfere?ĭepending on the intended purpose and person, the answers will vary considerably. How do you assess the results of the tools? The test image, which serves as the basis for all diagrams and comparisons, is this one: Probably the best known tools are in the ring: So I will compare the features of each tool and see where they have their strengths and weaknesses.Īnd of course I will test the actual JPEG compression capabilities and how the tools compare to Compress-Or-Die. There is only the optimal tool for a certain purpose. Unfortunately, such statements are made regardless of which settings have been made and without considering the consequences for the respective target group (photographer, web developer, ad developer etc.) or the respective application purpose (archiving, website presentation, printing etc.).īasically, THE tool does not exist. Usually they end with a simple: "It generates smaller pictures, so it's got to be better."
There are a lot of articles about online image compression tools in the net, most of them are very superficial.
0 kommentar(er)
