Automatically choose optimal JPEG compression in OpenCV? - opencv

Question

I'm wonder if there is a way to automatically choose a reasonable JPEG compression level in OpenCV?
The current JPEG sizes I'm getting are too large, and nailing it to a fixed value feels dirty. If I recall such features existed in image editors such as Dreamweaver. If there is no such features, i'm also wondering if somebody knows of an algorithm that is able to estimate this parameter without performing hard disk IO.
std::vector<int> params;
params.push_back(CV_IMWRITE_JPEG_QUALITY);
params.push_back(magic); //Want a way to estimate magic
cv::imwrite("my.jpg",image,params);

Answer 1

Unfortunately, to "optimize" JPEG compression, one would have to learn and apply many technical details about the JPEG compression. Because of this, many libraries do not offer the full suite of adjustment parameters. The 0-100 JPEG quality parameter is already a good compromise.
ImageMagick may have such functionality.
You are looking for a way to "automatically choose a reasonable JPEG compression level in OpenCV".
However, "reasonable" is subjective, and depends on the the image owner's perception of what features are important in the given image. This means the perception can be different for every combination of (different owners) x (different images).
The short answer
No, OpenCV does not currently offer this functionality.
The "sysadmin" answer
Look at OpenCV ImageMagick integration.
http://www.imagemagick.org/discourse-server/viewtopic.php?f=22&t=20333&start=45
The quick and dirty answer
Use method of bisection (0, 100, 50, 75, 87, ...) to search for a JPEG quality level that will approach a specified output file size.
Secant method may also be applicable.
Edited: Newton's method is probably not useful, because one cannot obtain the first derivative of the quality-file size curve without an analytical model.
Obviously this is too inefficient for practical every-day use, so it is not provided by the library.
If you want to use it, you have to implement it yourself with your own choice of techniques.
To avoid disk I/O, use cv::imencode which writes to memory instead of to disk.
The slightly longer answer
Although it doesn't implement this functionality, it is obvious that it is a nice feature to have.
If someone is willing to implement it with code quality good for use in OpenCV, OpenCV may consider accept it.
The yet longer answer
OpenCV uses jpeglib, or optionally libjpeg-turbo, and both libraries allow one to configure the technical details of JPEG compression.
Below I will focus on these technical details.
Read first: JPEG compression on Wikipedia
Of the JPEG compression pipeline, three of the compression steps can be configured by users of jpeglib or libjpeg-turbo:
Chroma subsampling
After the conversion from RGB to YCbCr, the chroma (color-carrying) channels: Chroma-blue and Chroma-red, are optionally stored in a lower resolution relative to the Luminance (Y) channel, also known as the Intensity or Grayscale channel, the latter is always stored at full resolution.
Most JPEG decoders can support these downsampling factors:
(1, 1) - no subsampling
(1, 2), (2, 1), (2, 2) - moderate subsampling, where one or both dimensions may be subsampled by 2.
(1, 4), (2, 4), (4, 2), (4, 1) - heavy subsampling. Note that the original JPEG specification forbids some of these combinations, but most JPEG decoders are able to decode them nevertheless.
Quantization table
Each JPEG image can define a quantization table for the "AC coefficients" of the DCT transformed coefficients
Each JPEG image can define a quantization table for the "DC coefficient" (i.e. the average value of the 8x8 block) computed from the DCT transform.
Quantization is the "lossy step" of JPEG compression. So, a technical user will have to decide how much loss (quantization) is acceptable, and then configure the quantization table accordingly.
Huffman table
Huffman coding is a lossless compression technique. In other words, if one could really spend time optimizing the Huffman coding table based on the statistics of the quantized DCT coefficients of the whole image, one can often construct a good Huffman table to optimize compression without having to trade off quality.
Unfortunately, the reality is more complicated, and such optimization is often not enabled.
It requires keeping all DCT coefficients in memory, for the whole image. This bloats memory usage.
Writing to the file cannot start until everything is in memory. In contrast, if a library chooses the quantization table and Huffman table up-front, without looking at the statistics of the DCT coefficients, then the library would be able to write to the file incrementally as rows and rows of pixels are being processed. Because libjpeg is designed to be usable in the lowest-denominator devices (including smart watches, and maybe your refrigerator too?), being able to operate with minimum memory is an important feature.

Answer 2

Sorry but there is no way to tell the size before you make compress the file. If you are not in a hurry, compress the image using different quality values and then select the best one.