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POHow to specify bitrate for JPEG compression?
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copied!<p>Is there a way to encode JPEG at a specific bitrate?</p> <p>Presently, I'm using imagemagick's <code>convert</code>:</p> <pre><code>convert Lenna-gray-100.jpeg -quality 1.1111 test.jpeg </code></pre> <p>Bitrate increases with quality, but it's non-linear. I want to control the bitrate explicitly. It doesn't have to be exact, but I want it reasonably close (within, say 0.1 bpp of the specified setting).</p> <p>Is there any encoder there that allows images to be encoded at a particular bit-rate? It doesn't have to be imagemagick, I'll take whatever works (preferably on Linux).</p> <p>A dumb way to do this would be to play around with fractional values to the <code>-quality</code> parameter until something close to the target bitrate comes out, but I'm hoping for a more elegant solution.</p> <p><strong>EDIT:</strong></p> <p>So I got bored and decided to do things the quick (but stupid) way.</p> <p>First, here's a graph of imagemagick's <code>-quality</code> vs bitrate:</p> <p><img src="https://i.stack.imgur.com/byOaY.png" alt="alt text"></p> <p>BTW, here's the image I used:</p> <p><img src="https://i.stack.imgur.com/Ay3s2.png" alt="alt text"></p> <p>So the change in bitrate is quite fine for lower quality values, but becomes coarse after about 80.</p> <p>Here's some sample code to encode an image at some target bitrate. I used OpenCV cause it allows for in-memory JPEG encoding (no I/O necessary). While I originally was going to mock this up with Python, unfortunately the Python OpenCV wrappers don't expose the in-memory encoding functionality. So I wrote it in C++.</p> <p>Lastly, I was thinking of using linear interpolation on the quality to get closer to the target bitrate, but since <code>cv::imencode</code> only accepts integer parameters, it's not possible to set a non-integer JPEG quality. The quality scale between OpenCV and imagemagick seems to differ somewhat as well, so taking the interpolated quality parameter from OpenCV and using in imagemagick's <code>convert</code> didn't work well.</p> <p>This means that the output bitrate isn't equal to the target bitrate, especially at higher bitrates ( > 1). But it's close.</p> <p>Can anyone suggest something better?</p> <p>Code:</p> <pre><code>#include <stdio.h> #include <cv.h> #include <highgui.h> #include <assert.h> #include <vector> using cv::Mat; using std::vector; #define IMENCODE_FMT ".jpeg" #define QUALITY_UBOUND 101 #define BITS_PER_BYTE 8 int main(int argc, char **argv) { if (argc != 4) { fprintf(stderr, "usage: %s in.png out.jpeg bpp\n", argv[0]); return 1; } char *fname_in = argv[1]; char *fname_out = argv[2]; float target; sscanf(argv[3], "%f", &target); Mat orig = cv::imread(fname_in); int pixels = orig.size().width * orig.size().height * orig.channels(); vector<unsigned char> buf; vector<int> params = vector<int>(2); params[0] = CV_IMWRITE_JPEG_QUALITY; int q; double bpp = 0.0; for (q = 1; q < QUALITY_UBOUND; ++q) { params[1] = q; cv::imencode(IMENCODE_FMT, orig, buf, params); bpp = (double)buf.size() * BITS_PER_BYTE / pixels; if (bpp > target) break; } cv::imwrite(fname_out, orig, params); printf("wrote %s at %d%% quality, %.2fbpp\n", fname_out, q, bpp); return 0; } </code></pre> <p>Compile and run using:</p> <pre><code>g++ -c -Wall -ggdb -I../c -I../blur `pkg-config --cflags opencv` -Wno-write-strings jpeg-bitrate.cpp -o jpeg-bitrate.o g++ -I../c `pkg-config --cflags opencv` `pkg-config --libs opencv` -lboost_filesystem jpeg-bitrate.o -o jpeg-bitrate.out rm jpeg-bitrate.o misha@misha-desktop:~/co/cpp$ ./jpeg-bitrate.out Lenna-gray.png test.jpeg 0.53 wrote test.jpeg at 88% quality, 0.55bpp </code></pre>

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