Wulf's Webden

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Stitch Up

I have now uploaded the watercolours from Friday’s Observation to Abstraction art class to my Flickr site: expect a series of pictures over the next few days. The particular challenge for this set was that all the images were quite large – approximately A3 in size – and I only have an A4 scanner so, before I could upload them, I had to stitch my A4 scans back into combined images.

The essential operation of sticking two images together is quite easy with image editing software like The Gimp. The key point is that you need to line them up so that there isn’t a visible disjunction where they join. For this, I have developed a fairly reliable protocol, as follows:

  1. Pick the master image and scale the canvas to make plenty of room for pasting in the other part. Create a new, blank layer.
  2. In the zone that is shared by both parts, identify a key feature and drop in a guideline perpendicular to the join on both images
  3. Look for other features that fall on the same line and which appear on both images. Does the second image line up the same as the first? If not, rotate to fix
  4. Drop a second guide on the base image that bisects the first one at the selected point of interest.
  5. Copy and paste the second image into the blank layer on the first one. Put the point of interest at the intersection of the two guides (using the mouse and then nudging with the keyboard – zooming in and using shift and the cursor keys works well).
  6. To fine tune, put the top image into ‘difference’ mode. Continue nudging until the overlap area is as black (matching) as possible.
  7. Reset the top layer to ‘normal’ mode and apply a layer mask with a graduated blend that merges the closely matching parts
  8. Flatten the result and finish off with cropping and enhancing as required. For my set, I used an aspect ratio of 7:5, approximating the original paper, and scaled to 5600 x 4000px (a nice, neat number) before using unsharp mask on a subtle setting to bring in a bit of the paper texture.

Phew! It is quite straightforward although it also takes a certain amount of time. However, the results are pretty good. As the images arrive over the coming week, take a look to see if you can pick out the join!

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