Hi Karim,
are we better doing perspective correction in GIMP or DT? Both programs
have intuitive options for this.
thanks
Dr Terry Pinfold
Cytometry & Histology Lab Manager
Lecturer in Flow Cytometry
University of Tasmania
17 Liverpool St, Hobart, 7000
Ph 6226 4846 or 0408 699053
________________________________
From: Top Rock Photography <ka...@toprockphotography.com>
Sent: Thursday, 17 September 2020 5:00 AM
To: Darktable Mailing List <darktable-user@lists.darktable.org>
Subject: Re: [darktable-user] Change aspect ratio? (non uniform scale)
Sorry, did a reply, but I do not think it went to the list. This was a reply to
Graham Byrnes, who wrote,โฆ
I think you'll find that this is what it does: if you correct for keystone on
one axis, then you apply a constant rescale on the other, you return to the
trigonometric approach you suggest. It would be possible to combine them, but
the 2nd axis effect is just a constant factor.
Then you could also add the loss of focus due to shifting the focal plane...
which is less likely to be popular.
Thank you. I was just going through my head as to what you said, and, at first
it made no sense. But looking at it from a geometric perspective, it is logical
that any โpixelโ expanded on the y-axis, ought to be expanded on the x-axis to
the same degree, and any reduced on the y-axis, likewise on the x-axis. Simple
arithmetics; no trigonometry needed. The thing is that the re-size is
non-linear, during the perspective correction, whereas correcting in The GIMP
later would be linear. (โฆAnd, ideally, the square pixel becomes a trapezoid
pixel ๐ ).
As for the focus issue, that was the problem in the darkroom. When we did that,
we had to stop down the aperture to achieve a deeper DoF, to keep the image
focused on the paper. Of course, film did not have โpixelsโ, but grain, and
grain is random, not in a neat little matrix. (That neat little matrix is the
one thing which still bothers me about digital photography).
I suppose that if the resolution was large enough, (and the final print small
enough), it would be less obvious, (which is one reason we tended to shoot
architecture at the lowest possible sensitivity, and the largest possible film
stock). ๐๐๐๐
Sincerely,
Karim Hosein
Top Rock Photography
754.999.1652
[https://docs.google.com/uc?export=download&id=1cTak7bAmck7Nq9AFC27jve5N8IUB8fuS&revid=0B4ZxH1wUdjk2aGFaakwwMHBycmp5R3hDd1BvYmdNTXJORXdZPQ]
I think you'll find that this is what it does: if you correct for keystone on
one axis, then you apply a constant rescale on the other, you return to the
trigonometric approach you suggest. It would be possible to combine them, but
the 2nd axis effect is just a constant factor.
Then you could also add the loss of focus due to shifting the focal plane...
which is less likely to be popular.
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