RAW preview in Editor window is blurry

I am running RawTherapee 5.8 under Linux. Quite happy with the software so far (I generally get really nice colors from the start :slight_smile: , but the RAW preview in the editor window is blurry / unsharp. I have made the following observations:

  • only RAW files are affected. JPEGs viewed in RT or other software show the same level of sharpness at the same size.
  • JPEGs exported from RT are fine.
  • the 1:1 / per pixel preview seems to be fine.

Is there some setting I may have overlooked? Is there a way to activate high quality rendering for the preview?

You aren’t overlooking anything :slight_smile:

RawTherapee “forces” you to go to a 100% (1:1) view to see some of the changes in detail or detail in general.

Those tools are marked with the 1:1 icon.

This from RawPedia:

Note that the effects of some tools are only accurately visible when you are zoomed in to 1:1 (100%) or more. These tools are marked in the interface with a “1:1” icon Zoom 1:1 next to the tool’s name.

The preview image is downscaled with a ‘tiled’ interpolation. This is a good compromise between speed and quality. There are only a few other ways we could scale: Scaling: GDK-PixBuf Reference Manual

Edit: updated with actual correct info

@Dr_Nick Which mode would you prefer?


Tiled (currently in use)



Thank you all for your quick replies! “Nearest” (first screenshot) looks much sharper to me than the other modes.

This is a well known feature of the nearest neighbour interpolator. It looks sharper to untrained eyes but it causes a lot of artifacts: the whole point of interpolation is avoiding those artifacts without losing too much sharpness.

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Well, from an academic point of view that is certainly true (in the end anything not viewed at pixel level will be interpolated to some extent) but it’s not a practical solution to the problem that the preview is definitely blurry. As a user I’d rather trade a few artifacts for a sharp picture.

I wonder how other RAW editors such as darktable manage to provide a sharp preview? To my untrained eye everything looks fine there…

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The biggest downside of ‘Nearest’ is that it distorts the image depending on the actual size of the preview. This is because entire rows or columns of pixels get left out when performing this type of scaling - it is inherent to the algorithm. You can clearly see this if you compare the text “FIDDLER’S” on the bottle in image 1 and 2.

The only feasible alternative I can think of, is to implement a custom rescaling, such as Lanczos or Sinc rescaling. That’s not the easiest of tasks and will likely slow down the preview rendering. Hard to judge whether it would be worth it…

Edit: for some reading Image Resizing for the Web and Email

Lanczos and Sinc are deadly slow if correctly implemented for large reductions in size, very likely to happen for image preview.

FWIW I had good results with a mixed approach. First I reduced the image by half (it is fast, just replace four pixels with their mean value) one or more times until the image was sligthly larger then required, then I used Lanczos for the final reduction.
The algorithm is rather fast and the result is good enough.

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