How to use the curves tool is the topic of http://meetthegimp.org/episode-7-the-curves-tool/

(I am sure you knew about this – but others are coming by….)

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.

You find the histogram under Windows/Dockable Dialogs.

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.

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 read the article at linux.com. Tomorrow I will give some tips and tricks.

Serge

I would like to reconstruction the GIMP logo,
and I don’t know how can i do that. Can you help me?

Thanks!

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

@Rootie & Serge: Thanks for the tips. I’ll try both ways. Guess next weeks theme…..

The panotools are great. I use Hugin, the GUI to it. Even for simple perspective correction I use it. It takes some time to learn but it all pays back.

As usual, thanks for the show!