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POExtracting DCT coefficients from encoded images and video
 

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    <p>Is there a way to easily extract the DCT coefficients (and quantization parameters) from encoded images and video? Any decoder software must be using them to decode block-DCT encoded images and video. So I'm pretty sure the decoder knows what they are. Is there a way to expose them to whomever is using the decoder?</p> <p>I'm implementing some video quality assessment algorithms that work directly in the DCT domain. Currently, the majority of my code uses OpenCV, so it would be great if anyone knows of a solution using that framework. I don't mind using other libraries (perhaps libjpeg, but that seems to be for still images only), but my primary concern is to do as little format-specific work as possible (I don't want to reinvent the wheel and write my own decoders). I want to be able to open any video/image (H.264, MPEG, JPEG, etc) that OpenCV can open, and if it's block DCT-encoded, to get the DCT coefficients. </p> <p>In the worst case, I know that I can write up my own block DCT code, run the decompressed frames/images through it and then I'd be back in the DCT domain. That's hardly an elegant solution, and I hope I can do better.</p> <p>Presently, I use the fairly common OpenCV boilerplate to open images:</p> <pre><code>IplImage *image = cvLoadImage(filename); // Run quality assessment metric </code></pre> <p>The code I'm using for video is equally trivial:</p> <pre><code>CvCapture *capture = cvCaptureFromAVI(filename); while (cvGrabFrame(capture)) { IplImage *frame = cvRetrieveFrame(capture); // Run quality assessment metric on frame } cvReleaseCapture(&amp;capture); </code></pre> <p>In both cases, I get a 3-channel <code>IplImage</code> in BGR format. Is there any way I can get the DCT coefficients as well?</p>
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