[Courses][gimp] Lesson 10: Stitching Panoramic Images

Akkana Peck akkana at shallowsky.com
Sun Apr 3 03:06:24 EST 2005

Sometimes a camera lens just isn't wide enough to show what there
is to see.  You stare at a wide, sweeping vista, but when you look in
the camera's viewfinder, it only shows a tiny piece of the view you
see with your eyes.

In this lesson, I'll show you how to shoot multiple photos, then
use GIMP to stitch them into a panorama.


Shooting panoramas means taking a lot of side by side shots that
overlap.  Start at one end -- I usually start at the far left -- and
shoot a picture, then rotate the camera a little to the right (not
very much: each image has to overlap with the previous one), shoot
another image, and so on.

The process of combining them into one wide panoramic image is
usually called "stitching", and you can make the job easier if
you keep in mind a few tricks when you're shooting the images:

- Overlap a lot: take lots of images.  Overlap helps because all
  camera lenses have some "warp" at the edges, and trying to overlap
  two edges that are warped in opposite ways can be challenging.
  Lots of overlap also means that you'll have more images than you
  really need, so if some turn out to be not as good, you can discard
  them without hurting the final image.

  Overlap at least 1/3 of the frame with the next frame (e.g. the
  right 1/3 of image 1 is the same as the left 1/3 of image 2), but
  more   is better.  I usually overlap at least 50%.

- Keep the camera level.  If you have a tripod or a rock or fence to
  set the   camera on, it will make it easier to make the panorama later.
  If not, just try hard to keep the horizon level in the viewfinder.
  GIMP can rotate images after the fact, but that makes things harder.

- Try to keep the exposures the same.  If you're shooting a pan that
  starts in dim light and ends in bright, it sometimes helps to meter
  off a spot somewhere near the center.  With most cameras, you can
  point at the spot in the center, press the shutter button halfway
  down (a light comes on to say the camera is ready), then move the
  camera back to wherever you're shooting your image.

- Zoom in a little.  You'll have to use more images to cover your
  scene, but there's a lot less lens warp when you're zoomed in, so
  it's actually easier to stitch a panorama that was shot that way.

Of course, none of these are absolute rules.  If you want to see the
difference, try shooting several sequences, and see what you prefer.
Most of you are probably using digital cameras, so shooting more
images costs nothing extra.


Once you get your images uploaded, start gimp and create a new, blank
image (either a white or transparent background is fine) big enough to
hold the final panorama.

How big is that?  Well, first decide what resolution you'll be using.
You may or may not want to use the full resolution of your camera.
Panoramas are really big, and since GIMP has to keep all the images
loaded, plus a bunch of other information, a big panorama can really
eat up memory and make the system slow.  If you have a lot of memory,
or you really need that resolution because you're going to print the
resulting panorama at a large size, then go for it.  If you don't have
much memory and your goal is something to stick on the web, you might
want to scale down.  For this lesson, I'll be using five images of size
609 x 457, but I'd normally work at one and a half to double that.
My original images:
and 81, 82, 83, 84 ... 5 images total.

Once you know your images' width, you can make an estimate of the final
canvas size.  The final width will be no greater than the sum of all
the image widths, in other words, imagewidth * number_of_images.
(For this course, I'm assuming you're making a horizontal panorama.
If yours is vertical, then swap width and height.)

In reality, though, if you overlapped at 60% of the image width, then
the final width is actually imagewidth * number_of_images * .6 plus
some slop in case you really overlapped by less than you thought.

The final height will be roughly the same as one image's height,
but allow a little slop there, too, since you may have shot some
images higher than others.  (If you used a tripod, you won't need
much slop here.)

I'm going to make a panorama out of these photos:
(why the odd numbers?  I don't remember. :-)
If I leave room for a 30% overlap (though I think I was closer to
half, assuming 30% will make the canvas wider so I'll be sure I won't
run out of room), the width comes out to 609 * .7 * 5.
That comes out 2131.5, but I'll be even more conservative and round
up to 2150 (I'll crop it later).  I'll use 500 for the height.

Once you know your canvas size, use File->New... to create a new image.
You're ready to start loading in your images.


Start with the leftmost image, and open it as a layer in your new
image.  Gimp 2 has File->Open As Layer.  In gimp 1, just open each
image, Copy, then Paste into the panorama image.  Remember to click the
New Layer button in the Layers dialog to make it a real layer, not
floating.  Use the move tool to move it all the way over to the left,
and roughly centered vertically.

Next, open the second image as a new layer, the same way you opened
the first; but this time, use the move tool to drag it to the right
so that it's approximately where it lines up with the first image.
Don't worry about getting it exactly lined up just yet.  Go ahead and
open all the images as new layers, and drag each one to roughly the
right position.

Once you have all the images loaded, in the right order and at
approximately the right places, it's a good time to save the image as
an xcf.  This will preserve all the layers, and the other effects
you'll add soon.

Here's a screenshot of my layers lined up:
(Obviously I overlapped a lot more than 30%!  My canvas is way bigger
than it needs to be.  But too big is okay.  I'll crop it later.)


Now comes the hard part: lining up all the images accurately.

Start with the first and second images.  Toggle visibility off for
all the other layers in the Layers dialog, so they don't distract,
 and zoom in quite a bit: you need to be able to see details.

Now we use a trick.  In the Layers dialog, there's a slider marked
Opacity.  Select the second layer (if gimp gave you a Background
layer on your new image, ignore it; "second layer" means the layer
for the second image in the panorama) and slide the Opacity slider
to about 50%.  The image should go translucent: although the second
image is on top of the first image, you can see the first through the
second where they overlap.

Now that you can see both at once, use the Move tool to move the
second layer around until it lines up as well as you can manage.
The two layers will not fit perfectly everywhere: even if you kept the
camera perfectly level, lens warp will make the images different at
the edges.  Try to make them fit each other pretty well around the
middle of the overlapping area.  Don't worry too much about the edges
right now.

When you get really close, you'll probably see it seem to "snap" into
focus, as ghostly double images become sharp.  When you get close,
sometimes it helps to put the mouse aside and use the keyboard: when
the move tool is selected and a layer is active, you can use the arrow
keys to move the layer around by one pixel at a time.

You may find at this point that one image is tilted relative to the
other one.  GIMP's rotate tool can fix that: rotate just the layer
that needs it, usually by only a few degrees.  The preview in the
rotate tool unfortunately doesn't handle transparency (and earlier
versions of gimp don't have a rotate preview at all), so you'll
probably have to rotate and undo several times before you get close.
Fixing rotations are the hardest and most frustrating part of
panoramas.  For your first panorama, don't panic if the images are a
bit tilted with respect to one another.  Try the rotate tool if you
want, but if it's too frustrating, just leave it.  We may be able to
compensate for it later.

Once you're reasonably happy with the alignment of the first two
layers, slide layer 2's opacity back to 100%, toggle layer 3's
visibility on, select it, slide its opacity to 50%, and line it up
with  layer 2 the same way you did before.  If you have more layers,
repeat until they're all lined up.

You can see that in my supposedly "lined up" panorama, all kinds
of things don't fit.  You may also notice that the last two images
had to be rotated, because they were so far off.  I really did a
bad job of shooting this panorama.  Good thing I have gimp!


To fix those non-fitting edges, Layer Masks, from Lesson 9, are just
the ticket.

We're going to use the layer mask to make layer 2 fade to transparent
near its left edge, where it overlaps with layer 1.  Toggle
visibility off on all the other layers, so it's easier to see
what you're doing.  Then select the gradient tool in the toolbox.

We'll draw a gradient on the layer mask, just like we did in Lesson
9.  Click on the edge of the second layer, and drag right to
approximately where you think the images might line up a little
better.  If you got really good alignment, then just drag in a little.
My images don't match very well, so I'm dragging in a lot.
Screenshot: http://shallowsky.com/images/gimpcourse/lesson10/gradient.jpg

At this point, that dotted yellow line at the layer boundary might be
getting in the way.  You can temporarily turn it off: View->Show Layer
Boundary.  (In gimp 1, use View->Toggle Selection.)  Like Show
Selection, this is confusing if you leave it off: you won't be able to
see which layer is selected.  So remember to turn it back on again.

It's also confusing to have a layer mask selected.  When you're done
setting up your gradient, click on the layer preview next to the layer
mask, so that the mask isn't selected any more.  Otherwise, gimp will
give you weird complaints when you try to save as jpg, and you'll get
confused (at least, I do) when you try to do anything with that layer.

Here's my layers dialog now:
Notice that the layer is selected, not the layer mask; and notice
that the layer mask preview is a little fuzzy on the left side,
where it fades to black.

With the first layer mask finished, my first two images look like this:
Compare that to what they looked like before (in the gradient screen
shot).  Quite a difference!

Now toggle the next layer visible again, add a mask, and draw a
gradient.  Repeat until you've added a mask for every layer but the
first one.  Here's what my image looks like now:


We're almost done.  You have a pretty decent panorama at this point.
But there may still be a few areas that bug you.

Sometimes, at this point, I discover that one of my images just isn't
as good as the rest.  Maybe it's not focused well, or too dark, or too
rotated.  Sometimes just toggling off one layer makes the image look a
lot better.  Don't feel like you have to use every image: that's one
reason to use a lot of overlap when you shoot, so you can throw away
bad images.  In my case, I found that turning layer 4 off improved the
image.  I didn't delete that layer: it's still stored in the .xcf
(don't forget to save the xcf file frequently, so you don't lose any
of your hard work) in case I change my mind later, but visibility is
turned off.  It takes up extra space (xcf files of panoramas with
lots of images can be quite large) but it's worth it.


But even if you turn off some images, you may still have areas that
just aren't quite right.  In my image, there's some confusion where
the drainage comes in from the lower left, where I can see two pieces
overlapping.  Also, I don't have a sharp edge on the right side of the
center canyon right near the middle of the image.  Sometimes you can
keep making bigger or smaller gradients until you get it right, or
angle the gradients (they don't have to be exactly horizontal); but if
that doesn't work, there's a better way.

Remember that a layer mask is just a white image that you can draw on
in black.  Anywhere you draw black, the upper image will become
transparent: effectively, it will be erased and the lower image will
take precedence.

So make sure your foreground color is black, and choose a drawing tool
(I'll use paintbrush) and a large fuzzy brush.  Then figure out which
layer is the one with the offending areas you want to make transparent.
Click on that layer mask in the Layers dialog to select it. Then paint!

After painting out those bad areas, here's my final panorama:


If you make a lot of panoramas and get tired of the "calculate canvas
size and create layer masks" steps, you can use my gimp plug-in,
"Pandora", to automate those parts.  It's at

HOMEWORK: If you have an appropriate set of images, or can make one,
stitch a panorama. Just a little one, two images, is fine.

If you don't have any suitable panoramic material handy, you can
do an alternate project instead: put two images together, and make
one image fade into the other using a layer mask.

NEXT LESSON: The panorama lesson is somewhat long and tricky,
plus I posted it a day late (I've been travelling, and haven't had
connectivity for the lasat few days ... I shot lots of panoramas,
though, so now I have a lot of stitching work ahead of me!).
So I'm going to allow two weeks for this lesson's homework, and won't
post a regular lesson next week.  Instead, I'll post some sources of
gimp information, tips, and tutorials, and I'll talk briefly about
GIMP plugins and scripts: where to get them and how to build them.

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