Exact preview render feature

It is known that some tools in RT require 1:1 zoom to be properly rendered in preview. And when some complex effects are in use such as a dynamic range compression, noise reduction or a wavelets levels it is hard to control the overal image appearance.
It is always take a time to save the image, look at that and then return to RT to apply some changes. And then repeat all that again.
So what if to add Exact preview render button which will initialize accurate image rendering without saving it to the file. Rendered image can be appeared as preview with zooming and panning (hand tool) available. And when user changes some settings, “exact preview mode” will be canceled and switched to regular one.
Applying such a feature can also improve a perfomance of zooming and panning because while image is prerendered there is no need to re-render complex effects during zooming and exploring.


In my understanding RT has such a function - at least sort of. In the editor just click the external editor icon. In my case I use Irfanview as external editor. Opening it with the rendered picture costs less than an additional second. After viewing, zooming etc. just go back to RT and make your further changes.

I agree, it is a weird requirement, having to zoom to 1:1 to adjust a setting (such as dynamic range compression) that has a global effect, which is impossible to judge when zoomed in. Having to export and open in an external viewer/editor just makes the process slower (even if it’s done with a single click) than doing it from within RT.

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It’s an old request: Button to preview fully processed image · Issue #2900 · Beep6581/RawTherapee · GitHub
I’ve added the 6.0 milestone to it, just to give it a little more exposure.


Art has something in the preferences now I think to enable this @agriggio might be able to elaborate if asked…looks like there maybe only for noise reduction??

I’ve tried external editor feature to preview rendered image in viewer. It looks like semi-solution.
First of all, it creates a new file everytime. It is not a big issue and you can delete them in the end.
Real thing that makes it not usable for accurate image tuning is that every new rendered image is opening in editor with another scale and position than preview image in RT.
It can be difficult to see what exactly changed in the image appearance if just a minor correction was applied.
It’s all because of the image is opening in the another place and with another zoom, not instead of previous the image inside the editor.

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Another tought. Exactly rendered image can also be used to provide correct histogram and clipping warnings. Scenaro when it can be useful is next.

  1. Apply Dynamic range compression.
  2. Turn on black and white toning.
  3. Turn lights clipping warning on.
  4. Try to set After curve white point right to the position where clipping appears on few pixels.
  5. Save.
    After that resulting image can be less contrast than it was on preview. You need to open that resulting image and setup curves again.
    Exact histogram from properly processed image can solve it.
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