But that's really not the interesting part of supporting dual-pixel RAW. Pedantically, yes, it can decode the fractional image. Langdon's data compression course (RIP)? :-) The CPU impact should be minimal-even my quick, hackish, naïve, non-vectorized implementation of most of this math ran 50 million times in under a second on a modern CPU, so the big processing hit should be tiny, too.Īm I missing something subtle? Are the differences much larger than I think they are? Or did nobody at Canon take Dr. I suspect that this relatively small amount of simple bit math would have been enough to reduce the dual-pixel RAW penalty enough to make it practical for everyone to just turn on dual-pixel RAW and leave it on. This will compress to a very small number of bytes because it will be all zeroes, so the data basically just says "30 million zero bits". Write the topmost bit of every pixel to a file using run-length encoding.(And the one-bit rotation is irrelevant if you do this.) But the real benefit of having mostly zeroes comes when you do bitwise run-length encoding.
This ensures that the left end of each number is almost all zeroes.īy making the leftmost digits be zero, various compression algorithms work better, including the lossless JPEG that they're already using, I think. Optional for lossless JPEG: Rotate each value one bit to the left so that the sign bit is on the right end.This ensures that small negative numbers are also almost all zeroes. For each negative value, compute the absolute value (or just invert all of the bits) and reset the sign bit to its original value.Similarly, you can subtract these from each other and L+R-(L-R) = L+R-L+R = 2R. You can add the resulting parts together and get L+R+L-R = 2L. It is also important to understand that, assuming you don't have too few bits to work with, this translation is lossless. What's important to note, at this point, is that except for the outliers where a pixel's values are very different on both sides (massively out-of-focus areas?), most of your values here should be very small positive or negative numbers, with many pixel values being nearly all zeroes or ones. Encode a second image as the difference between the left + right images.
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What were they thinking? They basically encoded (I think) the sum of the two images as the first image, plus one of the two images as the second (that's your one-stop difference). Compare two shots in Fine mode - taking the main frame from Dual Pixel raw (the left part of the picture below - the dual pixel raw and its histogram) and the "regular" Fine raw (the right part of the picture below - the "regular" raw and its histogram).
Here is how you can check that the main subframe of the dual-pixel raw is the same as a regular raw. This confirms that the difference between the main and auxiliary subframes is nearly 2x, or 1 stop, and that the auxiliary subframe can be used for highlight recovery (again, an additional 1 stop of highlights is preserved in the auxiliary subframe while being clipped in the main subframe), effectively providing one more stop of headroom in the highlights and the dual-pixel raw file for this camera contains 15 bits of raw data, if you consider main and auxiliary subframes together. Channel averages for the selection on the main subframe vs.
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