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Problem combining images into one

Aug 9, 2012, 19:09
Post: #1
Problem combining images into one
The attached files contain two pictures ("cat.jpg" and "nacelle.jpg") which I am trying to superimpose and combine into one, and a third picture ("balloons.jpg"), which serves as a reference only. Problem: how can the first picture, which shows a cat in his (partially visible) basket - but it could be any other animated creature, a face or an object, or even Wilber, if he agrees - be superimposed upon the second picture, that of a hot air balloon's nacelle? This second picture is a zoomed up detail of the third picture, "balloons.jpg" which shows a balloons' rally instants before lift-off (it may need to be zoomed up a bit for better viewing). The nacelle and its male occupant can be seen in the lower right part of that image. When superimposed upon it the cat should be positioned on the nacelle's edge and its tail should be turned towards the man.

I thought I could solve this problem by adding the first two pictures ("cat.jpg" and "nacelle.jpg") to an empty background image, using the cat's image as the top layer image. Unfortunately, after quite a few attempts I had to give up. Moreover, I was unable to give proper proportions to both images, as they are not at the same scale. I also failed to improve the sharpness which the nacelle's image has lost, partly due to its enlargment.

So, any help or advice would be very much appreciated.


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Aug 10, 2012, 15:09
Post: #2
RE: Problem combining images into one
(Aug 9, 2012 21:02)ofnuts Wrote:  Rule #1: start with the right material. As you found out you cannot really enlarge things if you want sharp images. Scaling down is always preferable, so in this case your starting image of the basket is very small.

As an aside, the problem all beginners run into is that not only do they miss a lot of practice in mitigating little problems, but they start piling them up by choosing sub-optimal images to start with. I'm not saying this to belittle you (we all fell for that at some point) but make you consider that when it look easy for others, it is also because, for a large part, they picked up the right images for the work (when they didn't shoot them on specifically for that purpose).

This said, to position the cat layer accurately, do the following:
  1. 1) Crop the layers to something sensible, that contains their actual contents (cat for one, balloon basket for the other) plus a bit of transparency.
  2. 2) Make sure nothing is selected (Select/None), the tools we are going to use have a different behavior when something is selected(*)
  3. 3) Move the cat under the basket in the layers list and make it the active layer.
  4. 4) Start the Scale tool (Scale). In the Tool options, there is a big "opacity" slider, set it to around 30%. There is also a "keep aspect ratio" box at the bottom. Tick it. Others options should be kept as defaults: Transform: layer (left icon), direction: normal, interpolation: whatever (Sinc(lanczos3) is best), clipping: adjust, Preview: image)
  5. 5) Drag the cat layer by the corners to reduce its size. You'll see that the first thing that happens is that you get a copy of the cat layer above the rest. This is the copy that you scale, and the opacity layer in the Tool options control its opacity. There is also a "Scale" dialog box that shows up. Keep an eye on it, you'll need it later.
  6. 6) Once the cat has a reasonable size, you can increase its opacity, and you can also also zoom in the image (zoom controls at the bottom of the image window, or Ctrl-mousewheel). You can also move the scaled cat layer by clicking the circle-cross at the center and dragging it. This should let you position the cat where you want.
  7. 7) Click the "scale" button in the "Scale" dialog once done. This make the "copy" layer disappear but applies the scaling to the original layer, so you must drag the cat layer back above the basket layer to admire your work.
  8. 8) If you want to give the cat a little tilt to follow the basket, use the Rotate tool (Rotate) the same way. The circle-cross in that tool is the rotation center, ie, the only point that doesn't move. All scaling/rotation will slightly degrade the scaled/rotated part so keep them to a minimum (one scaling+one rotation). If you eventually find yourself doing several, take your marks on the final result and try to redo them in 1+1.


(*) in many places in Gimp, selecting everything or nothing gives the same results. But not here.
Thank you for your reply. Your thorough guidelines solved the problem. As I kept the two images (the nacelle and the cat) at about 50% of transparency each, the final image was a superposition of the two semi-transparent ones. I therefore did a cut-and-paste (at a 100% opacity) to position the cat's picture over the nacelle, so as to have a unique, final image. But if there are better ways to do this, I'd be very happy to learn about them.
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