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cropped raw saving

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cropped raw saving
« on: 26 / October / 2015, 03:16:18 »
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I'd like to save some type of raw files that are cropped. They should contain the non-jpg areas which correspond to the centre cropped region.  The use for me is astrophotography.  When I take a picture of the moon or planet, there's only a small area I'd like to save.  The need for the crop is continuous shooting speed.

I can save 115 jpg/minute or 67 raw/minute, of black pictures, where the jpg was only 60kb.  This test illustrates the upper range of saving speed if size isn't an issue.  The SD card was 200x or UHS I, and the chdk raw write benchmark was 17343kb/s.  The average actual picture write speed worked out to about 12.4MB/s.

So this would mean the difference between 1 normal raw/sec or 2 cropped raw/sec, or possibly doubling the speed.  I like to take hundreds of pictures for stacking purposes.  I can think of some other uses too, just pictures of anything where you can trade a bit of resolution for speed.

The cropped DNG should be constructed with some of the non-active area where the black level is read, otherwise it should work just as well and seem like a real lower resolution raw.

Re: cropped raw saving
« Reply #1 on: 26 / October / 2015, 04:50:26 »
An I just had a better idea.  I would totally go for faster burst mode with smaller frames.  One of my cameras does an amazing 10 frames/s (for just 1s), but with 1/2 sized frames, it could be a short video clip except in raw.  Likewise you could skip pixels, but that doesn't look so good, unless you do superpixels processing, where each "pixel" saved is one complete bayer pattern, then there's no colour moire but just aliasing.

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Offline reyalp

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Re: cropped raw saving
« Reply #2 on: 26 / October / 2015, 15:55:31 »
FWIW, CHDK remoteshoot actually allows you to transfer a sub-image, but only with a vertical crop.  The DNG meta-data are not modified (so all the dimensions are the same as a full size raw), the client is expected to either pad the image (as chdkptp does) or adjust the meta data.

This is not available for raw saved to the SD card, though in theory it shouldn't be too difficult to support.

Supporting horizontal crop would be more difficult and often have worse performance. A vertical crop can be done in a single write call. If the rows aren't full width, you need one write per row or some kind of intermediate buffering (or maybe EDMAC tricks).

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One of my cameras does an amazing 10 frames/s (for just 1s), but with 1/2 sized frames, it could be a short video clip except in raw.
Raw is not currently supported in the modes that produce half size images (variously called low light, high iso, high speed) srsa did some research: http://chdk.setepontos.com/index.php?topic=10648.0

These modes are (usually?) binned, which with a bayer filter has an unavoidable impact on quality. However, I believe there would be some value in adding support for this in the future, for example the raw meter code could use it for faster timelapses.
Don't forget what the H stands for.

 

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