The lens distortion can be corrected using the “Lens Distortion” (under Filters->Distort) plugin

something odd: The video is 39 MB but this post says it’s 12 mb. ??

Serge, can you give me a pointer how to do that in Hugin?

Edit: found this: http://www.linux.com/articles/55174

Enjoyed it very much. Right at the end there was a throw away mention of PTlens a piece of software that I only became aware of today. Easy to use, corrects CA, and runs in Ubuntu when installed with Cross Over. No access to the help file however.

My comment is not related to show 37 but to first 3 or 4 … which I like very much.
I am just discovering your shows and enjoy them, so for the time being I have no suggestions to improve something. But I like you accent in English – I am not an native English speaker so it is much more easier to understand you than an American narrator – that’s usually the case for Photoshop tutorials.
Thanks for helping me and many others to learn Gimp in an easy way.
Mihai.

I would like to assure Mihai that I am able to understand Rolf far easier than many American narrators and I am a native English speaker.

Rolf,

The article you found gives a very good general introduction on how to deal with correction of barrel distortion and perspectiv distortion. The following one might also be useful.

http://hugin.sourceforge.net/tutorials/architectural/en.shtml

So I will not go into the workflow and GUI but instead give you some tips which might be useful. I will limit them to what is important for correcting only one picture, not a series of photos.

- The tab “Crop” can be skipped. The tab “Assistant” is optional; it guides you through the steps to be taken.
- Hugin will ask values for the HFoV (horizontal field of view) and focal length. Only the HFoV matters because Hugin uses this parameter in its calculations. The focal length is calculated automatically but is bogus for point-and-shoot cameras: Hugin presumes 35 mm film.
- An initial value of 40 degrees for HFoV is okay. It is not necessary to give an accurate value as Hugin will find it during its optimisation calculations. This is really the only value wich you have to supply. The others are not used or will be calculated by Hugin. (In fact, every value is a reasonable starting value of a parameter in mathematical formulae. Those parameter values are then varied within certain limits during the solution of the optimialization problem.)
- I add control points manually. Start with 4 or 5. I seldom use more than 10.
- Spread the control points and avoid the very border area of the photo if possible. Here lense imperfections are more severe and make the calculations for Hugin more difficult (I think here about the unsharpness and possible chromatic aberration, not the barrel distortion)
- Place control points at high contrast areas, like edges of houses or windows, sharp colour transitions, etc. Smooth areas make it more difficult for Hugin to optimize its result and can even cause nonsense results.
- If you use automatic generation of control points, check them. They are not necessarily correct!
- I normally start with optimization of only pitch and roll. This should already give a reasonable start. If not, then very probably something is wrong with the control points. It is not necessary to include yaw.
- After that I add view and optimize again. The calculated HFoV should stay in reasonable limits (I typically have 30-50 degrees) but is not necessarily the physical correct value. But that is not our goal; we want a nice picture. If the maximum deviation exceeds, say, 5 there is probably a problem with one or two control points.
- Then I add barrel. The resulting picture should now really look well and the barrel value should stay low (I typically have < 0.05)
- Distortion a and especially c are the fine tuning parameters and might even not be necessay. I always add one parameter at a time and check the result. Also these parameters should stay low (like < 0.05). At this point I sometime exclude view and/or barrel again. Especially when there are not many control points Hugin cannot always find a meaningful optimization with too many variables. If they are not excluded, the HFoV and barrel should only change slightly; if not, you should mistrust the result.
- x and y shift do not apply here.
- For “stitching” one picture, one should choose rectilinear projection (unless you are after some special effects)
- I use always the nona engine. It is way faster than PTStitcher and moreover, PTStitcher appeared to be not that stable on my system.
- Push always the button “Calculate Optimal Size” otherwise your image might get scaled.
- “Soft Blending” is useless for one picture. I do not use it anyway but do it manually in GIMP.

This is what comes into my mind now. I hope it helps and feel free to ask.

Serge

I just discovered Meet The gimp some days ago and already have downloaded all your screencasts. This is really awesome. The best resource about The Gimp by far. I’ve learned a lot about photo correction viewing your tutorials. Thanks a lot, Rolf

[...] Episode 037: The Two Minute Holiday Shot Edit [...]

A useful episode, nice to see some quick fixes but mostly nice to see them done quickly!

Thanks for introducing me to the perspective tool, I always wondered what it was useful for.

Not sure what has happened to the histogram tool with 2.6 as it’s no longer in the toolbox. The one from
the menu system doesn’t seem to work in the same way as the one you use pretty much every episode.

Sorry, I was confused .. I meant the Curves tool (which has a histogram, very useful for sampling).

I have since tracked down and used the Curves tool, some useful info in this episode about it.

I don’t think you used the Histogram tool, it seems to be pretty useless as implemented in GIMP.