Before the rush

Before the rush
by evan-pak

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This question already has an answer here:

I have the interpretation simplistically that you can make this by taking two pictures of high dynamic range picture and low dynamic range picture, and then merging. This way the HDR-picture should be invertible to the original picture.

I am interested in how these two pictures are or can be manipulated before the merge. I have an intuition that there are some linear filters. I want no tonemapping, since it is highly non-linear.

What can these filters be?

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marked as duplicate by mattdm, AJ Henderson, MikeW, John Cavan Nov 17 '13 at 23:54

This question has been asked before and already has an answer. If those answers do not fully address your question, please ask a new question.

This is not duplicate. This is completely different question. I am interested here on the implementation of HDR. I want minimal HDR implementation. I want to know if there exists any linear filtering before non-linear algorithms. Or if the implementation can be done with minimal amount of non-linear algorithms. – Masi Nov 16 '13 at 18:05
What you are asking seems to be exactly answered by Matt Grum's answer to that question. – mattdm Nov 16 '13 at 18:16

You have your terminology a little mixed up. "High dynamic range" refers to an image with a large distance between the darkest shades and the brightest highlights. The human vision system is better at coping with this than film or camera sensors, so in order to approximate that (or to go beyond!), one might take an image with a dark exposure (clipping the highlights but preserving shadow detail) and one with a higher exposure (blacking out the shadows but preserving the highlights) and merge the two. A merged file that contains all of this blended dynamic range is a HDR file, but, generally, there's no way to view this given the limitations of real hardware and printing technology. (Can you image a print glowing as brightly as the sun?)

So, instead, that HDR image is merged in some way down to a single image with a representable range of tones. This is often called an HDR image but really it is a regular-dynamic-range image which was made with HDR processing.

Even if the merge wasn't extreme, it can't be done because nothing tags the data as coming from one of the originals. If a pixel in the image is medium gray, is it because it was very bright in the dark exposure, or because it was dark in the bright exposure? There's no way to tell. It's like you have a glass of water with yellow dye and a glass of water with blue dye. Mix it together and you get green water, and you can't go back.

If you think you might want to, save the originals.

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Do you know the mathematical implementation of HDR? I want to have minimal HDR, so it probably means less non-linearity. – Masi Nov 16 '13 at 18:03

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