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Thread: PtGui users; question about getting more accurate when placing control points.

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Robert Pavich

Posts: 9
Location: Bridgeport, United States
Registered: 19 Mar 2013
PtGui users; question about getting more accurate when placing control points.
Posted: 27 Mar 2013 at 9:05 GMT
I've got PtGui and love it....but I did notice that it's tough to place control points accurately. I find that my cursor jumps around and goes back and forth over a spot and its' only by sheer luck that they are "exact"

I wish PtGui had a "nudge control point" feature, like if you hold down control and hit the left/right/up/down arrows it would nudge that control point and get really accurate.

Any tips about how to get better at it?
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Smooth

Posts: 4310
Location: Mount Panorama, Australia
Registered: 21 Jul 2004
Re: PtGui users; question about getting more accurate when placing control points.
Posted: 27 Mar 2013 at 9:10 GMT
Hold down the Ctrl and Alt keys and you will be able to position control points with greater accuracy.

Also you can zoom the images and also change the magnifying value in the settings.

Regards, Smooth cool
www.omnipix.com.au
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Robert Pavich

Posts: 9
Location: Bridgeport, United States
Registered: 19 Mar 2013
Re: PtGui users; question about getting more accurate when placing control points.
Posted: 27 Mar 2013 at 10:42 GMT
Thanks very much!
Just by chance...the PtGui support guy came back to me VERY QUICKLY with the same info...great company support.
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John Hawkins

Posts: 105
Location: Las Vegas, NV, United States
Registered: 11 Dec 2006
Re: PtGui users; question about getting more accurate when placing control points.
Posted: 28 Mar 2013 at 3:35 GMT
I wish Kekus had kept going with PTMac. It had a neat system where if you held down the shift option when moving the cursor, you could set the control points sub-pixel. It also had a great auto find where if you click in one image and then in the other, it automatically found the matching point in the second window also to sub-pixel. PTGui also has that, last time I checked but it wasn't sub-pixel which doesn't make as much difference now with the resolution of current cameras.
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DennisS

Posts: 1787
Location: Los Anglels, United States
Registered: 1 Sep 2007
Re: PtGui users; question about getting more accurate when placing control points.
Posted: 28 Mar 2013 at 5:26 GMT
updated: 28 Mar 2013 at 5:27 GMT
Accuracy of control point placement is a rather interesting subject.

I use the "maximum control point distance" just as a general guide to let me know how things are progressing.

You never need to be "sub pixel" accurate with your control point placement.

I have seen way too many situations where PTGui puts priority over accurate blending rather than matching up control points. Control points tell PTGui to "pay more attention to this area". PTGui does not try to peg the control points together, rather it puts priority on blending. Vertical control points use a different algorithm

If the pano stitches, ignore the numbers. I get some great stitches with a MCP distance greater than 5. Heck some have been as high as 10. We do not publish the stitching statistics. Who really cares as long as the pano looks good.

You can play the game of finding a control point pair that has the largest distance and adjusting one of them in order to get the "maximum control point" distance down. If you have a well calibrated rig and are not shooting blank walls you should be getting a MCP of under 3 after you do "delete worst control points".

I think too much empasis is being placed on sub pixel control placement. Your eyes are the final judge, not some numbers that nobody else will ever see. Poorly placed (or missing) control points will mess up a pano quicker than sub pixel placed control points will help.

Push and hold down the CTRL key while moving a control point. It slows down the mouse movement.
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John Hawkins

Posts: 105
Location: Las Vegas, NV, United States
Registered: 11 Dec 2006
Re: PtGui users; question about getting more accurate when placing control points.
Posted: 28 Mar 2013 at 6:16 GMT
Like I said, sub-pixel doesn't matter as much as it used to when we had lower resolution photos. With lower resolution photos, a pixel represented a larger angular distance and when blending was a simple feather, it made a difference. It still helps for an accurate stitch when you have a scene with thin straight lines. In an accurate stitch, they will remain straight (depending on the projection) and you won't have a wavy line that matches but isn't straight like it should be.

All said though, a good stitch is a function of a properly set-up rig, well spaced control points, distortion correction in the lens, and a smart blending algorithm.
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John Houghton

Posts: 3934
Location: Hitchin, United Kingdom
Registered: 17 Jan 2005
Re: PtGui users; question about getting more accurate when placing control points.
Posted: 28 Mar 2013 at 8:36 GMT

DennisS said:

You never need to be "sub pixel" accurate with your control point placement.

Provided you have a reasonable number of points, the control point placement errors resulting from rounding to integer values will average out to give an image alignment very little different to that which sub zero placement would give.

I have seen way too many situations where PTGui puts priority over accurate blending rather than matching up control points.

I don't know what you mean by this. Optimization with control points and blending are two completely separate processes performed in that order.

Control points tell PTGui to "pay more attention to this area". PTGui does not try to peg the control points together, rather it puts priority on blending.

The optimizer's job is to try to peg the control points together. Blending doesn't come into it. Likewise, blending knows nothing about control points. There may not even be any control points (e.g. when stitching with a template).

I think too much empasis is being placed on sub pixel control placement. Your eyes are the final judge, not some numbers that nobody else will ever see. Poorly placed (or missing) control points will mess up a pano quicker than sub pixel placed control points will help.

Sub pixel placement offers greater precision but it is doubtful whether the alignment accuracy would be significantly better, given the various factors that militate against getting perfect alignment. e.g. parallax (which cannot be completely eliminated), subject movement, imperfect lens correction, panohead setup and effects of sag due to arms bending.

I believe Hugin offers sub pixel placement, so it should be practical to carry out some comparative tests to see if the supposed benefits are realized in practice. Probably somebody has already done this.
Push and hold down the CTRL key while moving a control point. It slows down the mouse movement.

Remember that you can also nudge the cursor with the arrow keys.

John
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DennisS

Posts: 1787
Location: Los Anglels, United States
Registered: 1 Sep 2007
Re: PtGui users; question about getting more accurate when placing control points.
Posted: 28 Mar 2013 at 12:36 GMT
Actually I used the wrong word. Instead of blending I should have said alignment. As John pointed out. alignment and blending are two different processes. I have seen too many times where PTGui will place a priority on aligning matching areas over trying to get control points to match up.
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John Houghton

Posts: 3934
Location: Hitchin, United Kingdom
Registered: 17 Jan 2005
Re: PtGui users; question about getting more accurate when placing control points.
Posted: 28 Mar 2013 at 14:14 GMT

DennisS said:

I have seen too many times where PTGui will place a priority on aligning matching areas over trying to get control points to match up.

This still doesn't make any sense to me. The optimizer aligns control points only and doesn't know whether the images contain matching areas or not because it doesn't open the images themselves. It's not clear how PTGui could give priority to aligning matching areas.

John
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DennisS

Posts: 1787
Location: Los Anglels, United States
Registered: 1 Sep 2007
Re: PtGui users; question about getting more accurate when placing control points.
Posted: 28 Mar 2013 at 14:19 GMT
John,

I do not know how PTGui works internally either. All I know is that I have a couple of projects where PTGui has clearly ignored control point placement and aligned two images based on content. It does not happen too often but when it does, it can be very frustrating. I had to use Photoshop afterwards to get the images to align properly.

Dennis
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John Houghton

Posts: 3934
Location: Hitchin, United Kingdom
Registered: 17 Jan 2005
Re: PtGui users; question about getting more accurate when placing control points.
Posted: 28 Mar 2013 at 16:33 GMT
Dennis, The next time this happens, please let me have a copy of the images and project file and I'll maybe be able to explain what's going on. Or maybe you still have the files from the last occasion.

John
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DennisS

Posts: 1787
Location: Los Anglels, United States
Registered: 1 Sep 2007
Re: PtGui users; question about getting more accurate when placing control points.
Posted: 28 Mar 2013 at 17:45 GMT
updated: 28 Mar 2013 at 18:04 GMT
John,

I found a project that this was happening on.

It turns out that the version of PTGui I was using so many years ago was a very old one. I brought up the project in the latest version and PTGui does a much better job of stitching. I turned on Viewpoint Correction for the Nadir patch and the imgage looks much better. Maybe something internally to PTGui has changed or maybe I have just learned a lot about stitching since 2009. I can now get an acceptable stitch which still requires a bit of touch up in post processing (cloning out the visible parts of the tripod and such).

Dennis
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John Houghton

Posts: 3934
Location: Hitchin, United Kingdom
Registered: 17 Jan 2005
Re: PtGui users; question about getting more accurate when placing control points.
Posted: 28 Mar 2013 at 19:41 GMT

DennisS said:

.. maybe I have just learned a lot about stitching since 2009.

Dennis, I'm sure that's true!

John
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pboong

Posts: 4
Location: Singapore, Singapore
Registered: 3 Apr 2013
Re: PtGui users; question about getting more accurate when placing control points.
Posted: 3 Apr 2013 at 9:47 GMT
I too, had to use Photoshop to correct the alignment because I some how, still unable to get PT GUI to recognise the control point.

What I am trying to say is, despite having the vertical point already, there's some sorta slight off alignment on the building itself.

I tried putting the control point on windows, grill, building etc... especially the part where the two images merge together. but it still does not give me a proper straight line.

I even look at the max distance and delete them off after deleting the worst point.
Is it advisable to delete the max distance point? The best is less than 5?

I hope to get Very Good result, Good is acceptable for me because end of the day, I am also concern on the blending as well.

Thanks..
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John Houghton

Posts: 3934
Location: Hitchin, United Kingdom
Registered: 17 Jan 2005
Re: PtGui users; question about getting more accurate when placing control points.
Posted: 3 Apr 2013 at 10:06 GMT

pboong said:

I tried putting the control point on windows, grill, building etc... especially the part where the two images merge together. but it still does not give me a proper straight line.

You could try using straight line control points: type t3, t4, t5 ... . Put several t3 points along line 3, t4 points along line 4 etc. You must use PTOptimizer (special version available for download from the PTGui web site), as the normal optimizer does not support these points.

John
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