High Number of Key Points Causes Bad Alignment

[beratt54]

Hello everyone,

I was trying to figure out the effects of key point and tie point limitations on alignment. In medium settings with a key point limit is 40000 and the tie point limit is 0, I aligned my pictures and I got the results of 371/375 of pictures were aligned. However, when I increased the key point limit to 80000, 120000,....,240000 I observed that the number of aligned pictures decreased. I was expecting to have a result similar to this one:
https://www.agisoft.com/forum/index.php?topic=3559.0

I couldn't find any meaning to this behaviour, hopefully, someone can help me to understand it.

Thank you in advance,

Best.

Berat

[dpitman]

With regard to Key Points, the manual says

Key point limit
The number indicates upper limit of feature points on every image to be taken into account during
current processing stage. Using zero value allows Metashape to find as many key points as possible,
but it may result in a big number of less reliable points.

So, you are adding points, but they may be less accurate points.  The higher you go, the more "less accurate" points you will get and therefore photo alignment may go down.

Maybe try to add a tie point limit which, if I understand the manual, will act as a quality filter on the pile of key points and only select the points of highest quality leading to a better alignment.  I think at a certain point, alignment will be as good as it can get and adding more "data" can only make it worse and not better.  That is where the defaults come from. 

[beratt54]

With regard to Key Points, the manual says

Key point limit
The number indicates upper limit of feature points on every image to be taken into account during
current processing stage. Using zero value allows Metashape to find as many key points as possible,
but it may result in a big number of less reliable points.

So, you are adding points, but they may be less accurate points.  The higher you go, the more "less accurate" points you will get and therefore photo alignment may go down.

Maybe try to add a tie point limit which, if I understand the manual, will act as a quality filter on the pile of key points and only select the points of highest quality leading to a better alignment.  I think at a certain point, alignment will be as good as it can get and adding more "data" can only make it worse and not better.  That is where the defaults come from.

Hello dpitman,

Thank you for your reply, it actually make sense. I also tried with high quality and had the expected behaviour. I think high quality gives the enough alignment quality in that perspective.

Best.