![]() There are also different colour depths (palettes): Indexed color and Direct color. If you saved an image in a Lossy format over and over, the image quality would get progressively worse and worse. Lossy means the image is made (even) smaller, but at a detriment to the quality.Lossless means that the image is made smaller, but at no detriment to the quality.You should be aware of a few key factors.įirst, there are two types of compression: Lossless and Lossy. jpeg-bitrate.out Lenna-gray.png test.jpeg 0.53 G++ -I./c `pkg-config -cflags opencv` `pkg-config -libs opencv` -lboost_filesystem jpeg-bitrate.o -o jpeg-bitrate.out Printf("wrote %s at %d%% quality, %.2fbpp\n", fname_out, q, bpp) Ĭompile and run using: g++ -c -Wall -ggdb -I./c -I./blur `pkg-config -cflags opencv` -Wno-write-strings jpeg-bitrate.cpp -o jpeg-bitrate.o Int pixels = orig.size().width * orig.size().height * orig.channels() But it's close.įprintf(stderr, "usage: %s in.png out.jpeg bpp\n", argv) ![]() This means that the output bitrate isn't equal to the target bitrate, especially at higher bitrates ( > 1). 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 convert didn't work well. Lastly, I was thinking of using linear interpolation on the quality to get closer to the target bitrate, but since cv::imencode only accepts integer parameters, it's not possible to set a non-integer JPEG quality. While I originally was going to mock this up with Python, unfortunately the Python OpenCV wrappers don't expose the in-memory encoding functionality. I used OpenCV cause it allows for in-memory JPEG encoding (no I/O necessary). Here's some sample code to encode an image at some target bitrate. So the change in bitrate is quite fine for lower quality values, but becomes coarse after about 80. So I got bored and decided to do things the quick (but stupid) way.įirst, here's a graph of imagemagick's -quality vs bitrate: 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).Ī dumb way to do this would be to play around with fractional values to the -quality parameter until something close to the target bitrate comes out, but I'm hoping for a more elegant solution. ![]() It doesn't have to be exact, but I want it reasonably close (within, say 0.1 bpp of the specified setting). I want to control the bitrate explicitly. Presently, I'm using imagemagick's convert: convert Lenna-gray-100.jpeg -quality 1.1111 test.jpegīitrate increases with quality, but it's non-linear. Is there a way to encode JPEG at a specific bitrate?
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