[argyllcms] Re: scanin and perspective distortion
- From: Gerhard Fuernkranz <nospam456@xxxxxx>
- To: argyllcms@xxxxxxxxxxxxx
- Date: Wed, 06 Feb 2008 01:46:28 +0100
Guy K. Kloss wrote:
> Hi,
>
> I've experimented a bit with the scanin tool for reading characterisation
> targets. As the use targeted case here involves digital cameras (still &
> video) the capturing of the target is not as trivial as for e. g. scanners.
>
> Of course, the image needs to be captured in a way that no specularities or
> any kind of glare is present on the target. This can be achieved by either
> changing the positioning of the camera, the illumination or the target. For
> many cases however the characterisation is to be performed in the geometric
> conditions as given by a fixed setup in a fixed location (towards windows,
> lamps, etc.). So the only way to remove glare is to move or tilt the target
> relative to the camera.
>
> Doing this I have discovered the following, which is not an issue for
> scanners
> as input devices. The scanin tool seems to be relatively robust towards pure
> rotation and (uneven) scaling as long as it is along the target's axes. For
> testing I've created a few test samples. These are the problematic
> circumstances:
>
> * perspective distortion
> * pin cushion type distortion
> * rotation with uneven scaling
> (or scaling along a non-main axis of the target)
>
> The issue is now, that any image distortion that leads to a non-rectangular
> shape of the target potentially endangers the characterisation, as pixels
> outside the patches (or belonging to other patches) will be evaluated. This
> means that the target MUST remain orthogonal towards the visual axis of the
> camera, with only limited pin cushion distortion of the optical system.
>
> Any clues on solving this problem either in software or in handling the test
> setups? If bad comes to worse I've got to dig deeply into the OpenCV tricks
> box and try to undo certain distortions of the image before the
> characterisation process. However, I'd rather like to avoid that to not
> induce a higher error than necessary.
I think you should not overestimate small errors introduced by
correcting distortions with an image editing program, particularly if
you do it with 16 bits per channel, in a linear light space (raw RGB
with gamma 1.0). When capturing a target with a camera, there are IMO
other typical sources of error with a significantly larger contribution
to the total error (for instance uneven illumination, natural and
optical vignetting induced by the lens, still some residual glare).
Regards,
Gerhard
Other related posts:
