scale bar size keeps changing from my input

[renevg]

Hi,
I tell metashape the distance between two targets on a scale bar is 100 mm. I change both markers to control points by enabling the markers and the scale bar in the reference pane/source tab.  Still metashape tells me it's got distance errors.   How can I fix this? I thought creating control points would do this.
Thanks,
René

[renevg]

anyone?

[jkova96]

Hi renevg,
I know how frustrate can be when no one answers on your message and you hope (or thought) that this is the right place to post questions or so.
Believe me or not, but before few days I found that someone post questions on this forum, and later he or she - replies to himself or herself cause it found solution to issue that has not been solved at the start. This is so SAD...but it's true... that people used to 'selftalk'.

Now, from my introduction, let's get to your issue.
I can say that, from my experience I first "set up" coordinate system in order to avoid any problems either with check points precision or with scale bars.

For setting up coordinate system, it's local coordinate system, I use 3 points which lays on flat surface (it can be floor, it can be surface of the table - anything that have flat surface in your model).
Then, I use created excel table, which I can share with you in order if you still face with that problem. From this 3 points I create custom coordinate system: first point is origin of that system, second point defines X axis and third point defines Y axis from that origin. You don't need 4.th point for defining Z axis cause Z axis is perpendicular to XY plane. If you measure distance between these points, excelent  You can then use your measurements as values for each point which defines custom coordinate system. I did not mention, but these 3 points should form right-angle triangle, but If you can't predict which would fall 3.rd point - I also defined in excel table calculation of that 90° spot for 3.rd point. You can also done that for yourself if you know linear algebra.

NOW...Comes KEY PART once you determine coordinates of points defining your coordinate system you SHOULD check these points with check marks then hit UPDATE/TRANSFORM.

After this - You can freely add as much as possible scale bars, and if you have good measurements and good reconstructed 3D model there's no fear for bad results or don't know possible Agisoftmistakes.
Issues occures when you first define scale bars, then coordinate system, cause I'm not expert but I might think that issue lays - that Agisoft already give these points values without applying scale bars. When you apply scale bars then points don't colsolidate well, and that's the reason for high errors.

[renevg]

hello,

I 'm using a turntable with fysical scalebars with targets, 10 cm apart. (or any given distance for that matter). So size is known. Targets get recognized. I create scale bars from them. I check mark the scale bars and update transform.
I guess what I'm asking is: is there a way to absolutely fix the 10 cm distance, like one can fix the camera calibration with 'fix parameters'?
I know we can change the  marker accuracy(pix) and the scale bar accuracy (m) in reference settings, and accuracy settings in the reference pane. and by check mark the scale bar it has no errors. I have quite a few scale bars from targets.

thanks for your answer.
René

[jkova96]

Hi renevg,
Sorry for eventually misunderstanding?
Did now everything works fine for you?
I thought that something is wrong with your results, at least when I read all your forum question - I came to this conclusion
I'm so sorry If I were overloaded you with mine approach, how to hande this.
I did not count that you have Agisoft markers on your turntable setup.
I saw that you can do so much things in "Camera Calibration" dialog.
I saw also one option, if my vision was good, with scale bars also.
Don't know if accuracy is in question or something else.

Yeah, maybe I'm not right guy to told you what you want to hear.
To be honest with you, Now your question intrigues me much more...
Cause I want to know too if possible to do so, what you want to do.

I know that you can use created scripts from Agisoft LLC, from GitHUB. Don't know If I saw something similar to your request.
I'm really sorry cause I'm really useless when your task comes in place.
It would be really nice that someone from Agisoft Support sees this post cause I think they know the best.

[renevg]

Hello
@ jkova96:
thanks, for your sharing of knowledge. I should have been clearer. Indeed a script might be the answer.
best wishes,
René

[tazzo]

Hi, can you share a sample project showing the issue or some screenshots?

[renevg]

Hi
here is a screenshot of what I mean. I've got 3 control scale bars. Shouldn't they be without error because it's a given distance? the update button has been pressed.

[tazzo]

You can set the scale bar lenght and use it to check the model or as input data depending on the checkbox.
The model may be slightly different, this is normal. Some time ago I had a 0.1mm error, I then realised Metashape was right and that my ruler was a bit off.

[renevg]

thanks tazzo, but
when using control scale bar (so checked), the scale should be fixed (as in fixated) for that scale bar. The model should alter it's shape conforming to the control scale bars. That's the whole point of having control scale bars. But I must be making a error in reasoning? just like to know.

[tazzo]

when you use the scale bar as input, this is not the only input so will merged with other inputs (ex. points from images) and you may get an error to have the model consistent

[jrp]

If I understand right, you are concerned that -0.000114 and 0.000078 etc are not 0.

This data looks exactly how I would expect a good clean data set with well used scalebars to look.

Every measurment of any real world object has errors, including the measurment of scale bars.

Metashape makes a model of the scene, then takes an average of the measurments it takes of the scale bars, then scales the model to that average. Note that some of the erros are positive and some are negitve, and adding them up, they all come out at around 0.

The largest error shown here is around 1 in 1000 which is better than I could do with a ruler and my eyes; and may be better than the printing process the scales were made with.