Monday, October 10, 2016

Non-destructive Lab editing with GIMP and the PhotoFlow plug-in

Since a while PhotoFlow is also available in the form of a GIMP plug-in. It means that PhotoFlow can be used directly within GIMP, to open and process RAW images as well as to apply non-destructive editing to any GIMP layer.

The most simple way to use GIMP in combination with the PhotoFlow plug-in under Linux is to download and run the AppImage package that is hosted on the pixls.us web site. This screencast explains the details of how to run the GIMP AppImage and briefly introduces the tools that are provided with it.

One of the possibilities offered by the PhotoFlow GIMP plug-in is to apply non-destructive edits in Lab colorspace to any GIMP layer. The Lab colorspace (or better CIELab) is an alternative representation of the pixel data that, unlike RGB, separates the luminance information from the color information. It is therefore possible for example to adjust the luminance of the image without changing the color components, or to adjust the color saturation in a way that is more natural and pleasing than in RGB.

Let's see how this works in practice.



The starting point is an existing image opened in GIMP. To run the PhotoFlow plug-in on this image, go to the "Filters" menu and select "Filters -> PhotoFlow...":


If needed (if the selected layer was already created with the PhotoFlow plug-in), select "Create new" in the little dialog that pops-up when the plug-in is started:


The image in the GIMP layer is then loaded into the background layer of the PhotoFlow plug-in, and the editing can start:


The first step is to add a "colorspace conversion" layer to convert the pixel data to Lab. For this, one has to click on the small red button at the left of the background layer, and then select the "color profile conversion" tool from the "color" tab of the dialog that has been opened:

 

Once the OK button is clicked, a new layer is added above the "background" one. The tool associated to this new layer gives the possibility to convert the pixel data to a new colorspace. The Lab option is chosen in this specific case:


At this point, any tool inserted above the "colorspace conversion" layer will be working in Lab colorspace. For example, the "curves" tool gives the possibility to separately adjust the curve of the "L", "a"and "b" channels. Here I have applied an S-shaped curve to the "L" channel, aimed at increasing the contrast and slightly reducing the brightness of mid-tones:


Adding contrast in the "L" channel has often the tendency of simultaneously reduce the overall saturation of the image, like in this example. To compensate this effect, I added a "basic adjustments" layer above the curves, and increased the saturation a bit. When working in Lab colorspace, the saturation is adjusted by multiplying the "a" and "b" channels by a constant proportional to the value of the slider: the constant is > 1.0 if the saturation adjustment is positive, and < 1.0 if the adjustment is negative:


Most of the tools in PhotoFlow are able to process pixel data in Lab colorspace. Moreover, several tools can restrict their action to a specific channel. For example, here I have applied a final sharpening step that only processes the "L" channel and does not modify the color information:



Once the editing is finished, the plug-in can be closed by clicking on the "OK" button, and the image is transferred back to GIMP as a new layer:


When sending the image data back to GIMP, the PhotoFlow plug-in also stores the editing configurations as meta-data attached to the newly created layer. This allows to re-open the plug-in and further tweak the edit if needed. To do that, one has to select the layer created by the plug-in, start again the plug-in from "Filters -> PhotoFlow..." and then select "Edit current" in the pup-up dialog:


This will restore the plug-in in exactly the same state as it was when the layer was modified the last time:


Any further modifications will be saved again in the GIMP layer together with the updated image, and will be available for further tweaking...

8 comments:

  1. I'm looking to use the Whi-Bal plug-in with Gimp,does this work in Photo Flow.I know it can be used in Lightroom but I refuse to pay for it.

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    Replies
    1. Could you provide a link to the plug-in web page? Also, are you interested in fixing the white balance of RAW or Jpeg images (it makes a lot of difference...).

      Delete
  2. This comment has been removed by the author.

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  3. I have located this link by Image Resource that might shed some more light on my request.I did not however realize that the plug-in was a third party link,I was under the impression it came from Whi-Bal.You will see after reading the article that it is for RAW workflow,although it can be used for JPEG.
    http://www.imaging-resource.com/ACCS/WHB/WHB.HTM

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    Replies
    1. For RAW workflow, you do not need the plug-in: you can open the reference shot containing the gray card, use the "spot WB" option in photoflow and click anywhere on the gray card to make it neutral, then either note down the WB multipliers or copy the "RAW developer" layer.

      For the next RAW image of the same series, either open the "RAW developer" layer and enter the WB multipliers from above, or delete the "RAW developer" layer and paste the one you copied from the reference shot...

      If needed, I can record a short screencast to explain the whole procedure (it is much faster to do than to describe), but I would need a couple of RAW shots, one with the gray card and another without from the same series.

      Delete
  4. I have located this link by Image Resource that might shed some more light on my request.I did not however realize that the plug-in was a third party link,I was under the impression it came from Whi-Bal.You will see after reading the article that it is for RAW workflow,although it can be used for JPEG.
    http://www.imaging-resource.com/ACCS/WHB/WHB.HTM

    ReplyDelete
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  6. Thanks for the information and links you shared this is so should be a useful and quite informative!

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    ReplyDelete