Hugin should support 'zeronoise' output

[Bruno Postle]

This is a feature that should be easy enough to add, we figured all this out a while back but I haven't got around to writing it up.

'zeronoise' is a tool that combines bracketed exposures to produce output with reduced noise in dark areas, Hugin could do the same thing (but much better) as an alternative to the usual enfused output. You can reproduce the technique like this:

Shoot a bracketed stack (or more than one) with the shortest exposure the 'right' one, and with additional overexposed shots to capture dark areas.

Remove any 'blown out' areas of the overexposed shots by setting 100% transparency to these areas, this is very important.

Photometrically align them in Hugin and generate 'Normal' output with individual remapped images, i.e. you will end up with a series of photos that all have the same 'exposure', except they started as differently exposed photos. Some shots will have more noise that others, and others will have transparent areas where there should be highlights.

Now run them through enfuse. It turns out that enfuse is _very_ good at picking the lowest noise data from each shot, you will end up with a photo that looks just like a normal photo except with hardly any noise.

To do this, Hugin would need:

A nona option to automatically remove blown out areas from input photos when remapping, actually I suspect this would be generally useful when using enfuse 'normally'. Note that a 'blown out' area may only be saturated in a single channel.

An additional output option that does exposure correction like 'normal' output, but which uses enfuse to merge stacks like the existing 'exposure fusion' output.

[rew (r-e-wolff)]

Bruno, why is this different from what we do now with enfuse anyway?

[Bruno Postle (brunopostle)]

Normally with enfuse you provide differently exposed images to generate output that has a combination of the best exposed bits - To do this Hugin turns off any exposure correction. The result is not a normal photo at all, the exposure varies in different parts of the image.

With 'zeronoise' you supply identically exposed images with different noise levels - To do this you take bracketed photos and exposure correct them before merging. The output has less noise, but it is otherwise a 'normal' photo.

[tmodes (tmodes)]

> To do this, Hugin would need:
>
> A nona option to automatically remove blown out areas
> from input photos when remapping,

Implemented in changeset f08e456e35d3. Call nona with option --clip-exposure to mask all under and overexposed areas automatically.

> An additional output option that does exposure correction
> like 'normal' output, but which uses enfuse to merge
> stacks like the existing 'exposure fusion' output.

I decided against implementing a hard coded version for this case. Instead I added in changeset e742ac9250cd an "user-defined output" mode. This is more flexible and allows also to add other variants. The zero noise output is added as example.
Further modifications can now added more easily.