How to subtract photos with GRIP
This technique is suitable when two photos have been taken with the same exposure settings and with the camera fixed (eg, on a tripod) but something in the scene has moved or changed in some way. Fixing the camera ensures that an unchanging background will be the same in both photos (except for possible noise if the exposure was long or a high ISO setting was used). Subtracting the photos will produce an image in which only those things which changed will be seen.
- Fix the camera and use manual settings to take two photos. Automatic settings might be different for each image, so manual is best.
- Open both images in GRIP by using the Open option on GRIP's main File menu. The images will be in two separate windows with their own menus.
- If the images were taken in RAW mode see how to open RAW images.
- If GRIP's main window is hidden by image windows bring it to the front by typing Ctrl-G.
- On GRIP's file menu select "List and select images". A new window will be displayed, containing a table of all the images which are now open. This window has its own menu bar.
- Select both images in the table. This can be done in various ways:
- Hold the Shift or Ctrl key down and click on each row in the table.
- On the table's Edit menu, choose "Select all" (assuming only our 2 images are open).
- Type Ctrl-A (assuming only our 2 images are open).
- When just two images have been selected, the table's Combine menu will show that options "Subtract selected images" and "Reverse subtract selected images" are available. Select either of these and see the result of subtracting either the second image from the first (result in the first image's window) or the first from the second (result in the second image window). In both cases those pixels which have not changed will appear as mid-grey (level 127 in each channel if the image was 8-bit, level 32767 in each channel for 16-bit images). Other pixels will be brighter or darker than that depending on how different they were between the two images.
- Reverse subtraction is only a convenience. You could get the same result by using normal subtraction and then selecting Invert from the Levels menu of the result image.
- If you want the background of the subtraction to be black (zero in every channel) you could then go to the Levels menu, select Curves, and use the V-shape option in the drop-down list. The V-shape is specifically available for this purpose but it may have other uses. Doing this "rectifies" the subtraction so that all changes are brighter than the background and you will no longer be able to tell in which original photo a given pixel was brighter.
- Save the result by using the "Save as" option of the Image menu of the result.
As with all these things, experiment a bit with the steps given here to see which sequence produces the best result.
The original 2 images:
The result of subtraction (notice small differences in the foliage, probably due to a breeze):