Wednesday, December 06, 2017

documented GIMP workflow

Occasionally I only have GIMP at my disposal when trying to do LRGB processing on my images. I use it infrequently enough that I forget steps and specific commands. Here I quickly document the workflow.

These notes pertain to GIMP version 2.8 (which is 8-bit) running on an old version of Windows (32-bit).
  1. open all the TIF files (clearing the 16-bit warnings)
  2. change to full-colour mode
    1. pick any one of the images, e.g. the blue data
    2. choose Image, Mode, RGB
    3. rename layer: e.g. blue
  3. copy image data into layers (green, red, and luminance)
    1. go to other image window, e.g. green
    2. select entire frame: choose Select, All or press Ctrl-a
    3. copy: choose Edit, Copy or press Ctrl-c
    4. switch to target (RGB) image    
    5. paste: choose Edit, Paste as, New Layer
    6. rename layer: e.g. green
    7. close source image file (i.e. close the green data file)
    8. repeat for all source frames
  4. prepare to align
    1. except for the bottom layer, set each layer to partial opacity:
      click in middle of Opacity bar or drag or enter 50%
    2. zoom in
      1. click magnifying glass in Toolbox
      2. click in image (to zoom in)
      3. hold Ctrl key to zoom out
  5. align layers
    1. click layer to move
    2. activate Move tool:
      choose Tools, Transform Tools, Move or press M or click 4-headed arrow in Toolbox
    3. move layer
      1. drag with mouse
      2. nudge with arrow keys
    4. repeat for each layer
  6. save a version of the file *
  7. crop out the edges
    1. click Rectangle Select Tool from Toolbox or press R
    2. drag within the image
    3. choose Image, Crop to Selection
  8. colourise each layer
    1. click a layer
    2. choose Color, Colorize...
    3. set the parameters as necessary
      1. set Hue to required value, e.g. 120 for green
      2. set Saturation to 100
      3. set Lightness to 50
    4. repeat for each layer
  9. blend top two colour layers
    1. click a layer
    2. set layer mode to Screen
  10. set the luminance layer to Soft Light
  11. save a version *
  12. carry on, apply levels, curves, saturation, masks, etc. 
These steps are provided as an alternative to using Photoshop.

* Remember that GIMP does not support non-destructive editing of layers so consider layer duplication and saving versions of your file as you work.

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