Lightroom Classic Transform-Scale function

[John32484543uboq]

I have received "definitive" inputs from several professional photographers (including one popular You-Tuber) about the Transform-Scale function in Lightroom Classic.  However, these inputs are contradictory.  Question:  Is Transform-Scale the same as cropping?  If so, how is the pixel count maintained?  Or, is there some computational interpolating being done when you scale?  If there is, indeed, some "magic" going on, does it make sense to scale first, and then fine-tune with cropping?  I cannot find anything which addresses this question directly on the forums, so a technically accurate answer would be much appreciated.  Thanks.

[JohanElzenga]

No, they are not the same. Upscaling is an interpolation method, which crops into the image without changing the pixel count. Cropping changes the pixel count. I don't see the usefulness of this however. Anything you do in Lightroom is non-destructive. The actual copping/upscaling is only carried out when you export a copy. On export you can scale as well however, so you could export a cropped image and upscale it to the same original pixel count as when you upscaled it in the Transform panel. Are the interpolation methods the same in that case? I don't know for certain, but because everything happens only in the export process, I would be very surprised if that were not the case.

P.S.If you are interested in upscaling your images, you should have a look at the Enhance - Super Resolution option. That is an AI driven method, so that is definitely different from upscaling in Transform...

[johnrellis]

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I've idly wondered over the years how Transform > Scale and cropping differed computationally and in their results.  So I did an experiment, and the results indicate that when you examine exported (rendered) files, the differences are indistinguishable until you pixel-peep at zooms greater than 200%.   

Download this catalog to follow along with what I did:

https://www.dropbox.com/s/jv5ctxbjzspxiiu/scale-versus-crop.2023.09.23.zip?dl=0

1. I made two copies of a 6000 x 4000 raw, crop.nef and transform.nef.

2. I applied a 4800 x 3200 crop manually to crop.nef by saving its .xmp sidecar, editing these lines:

   crs:CropTop="0.1"
   crs:CropLeft="0.1"
   crs:CropBottom="0.9"
   crs:CropRight="0.9"

This moves the four edges inward by 10%.

3. I set Transform > Scale of transform.nef to 125.  

4. Observe in LR that crop.nef and transform.nef appear identically cropped.

5. I exported both files as TIFFs at size 4800 x 3200 and opened them as layers in Photoshop.  When you zoom in and enable/disable the top layer, the differences are visually indistinguishable until you zoom above 200%.  The result is stored in difference.tif.

6. I set the blend mode to Difference and flattened the image. The result is nearly black, indicating the pixels are almost equal. The Levels histogram quantifies the differences: 

7. I repeated steps 5 and 6 but exported at size 6000 x 4000 instead.  The layers are stored in difference-2.tif. The two images are again visually indistinguishable at 200% zoom and less, and the pixel differences are even smaller:

Analysis

LR defers generating actual image pixels until it needs to display an image or export it.  It represents edits as a "pipeline" of transform operators that are applied at render time.

In the first experiment (steps 1-6):

- crop.nef is cropped by [0.1, 0.9, 0.1, 0.9] to an effective size of 4800 x 3200 and then rendered by export at 4800 x 3200. No upscaling or downscaling is required.

- With transform.nef, the Transform > Scale setting scales the image up by 1.25 (to make the dimensions 7500 x 4880) then crops it by [0.1, 0.9, 0.1, 0.9] to constrain its dimensions to the original 6000 x 4000.  Export then renders it at 4800 x 3200, downscaling by 1.25. 

Naively, I think the rendering engine could notice that transform.nef is first upscaled by 1.25 and then downscaled by 1.25, the two cancelling out, and the results should be identical.  But clearly some pixel computations between crop.nef and transform.nef are slightly different, perhaps having to do with the very complicated stages of the Camera Raw rendering pipeline.

In the second experiment (step 7):

- crop.nef is cropped by [0.1, 0.9, 0.1, 0.9] to an effective size of 4800 x 3200. Export then upscales it by 1.25 to a size of 6000 x 4000.

- With transform.nef, the Transform > Scale setting scales the image up by 1.25 (to make the dimensions 7500 x 4880) then crops it by [0.1, 0.9, 0.1, 0.9] to constrain its dimensions to the original 6000 x 4000.  Export then renders it with no up- or downscaling at 6000 x 4000.

Here, both images are upscaled by 1.25 and cropped.  The difference between the two images is quantitatively less, but it's not zero.  Again, I'm not sure why there are any differences.

But practically, you can't distinguish the difference between cropping and Transform > Scale.