# Match on three points ...

Bug #1736722 reported by Houbrechts Didier
This bug affects 1 person
Affects Status Importance Assigned to Milestone
Invalid
Undecided
Unassigned

### Bug Description

This is for the wishlist ...
I mainly use Inkscape to compare insects specimens on picture with shapes/venations already known...
For that I need to transform the shape using 3 points that I already know on the picture using Object > transform > matrix.
I have looked for an already made extension for Inkscape and found nothing.
It's like changing the plane of an object from one to another knowing 3 points of each plane !
I join a sample to make you understand ...
I open the picture (on the left of my drawing and using bezier trace some interesting points and 3 very important points (called K,L,M on this sample) I already know I have them on shapes/venations (called A, B, C),
Then I create a new layer and import my shape (with the points A, B, C)
I need to calculate the affine transformation for A,B,C to match K,L,M
For this I read the coords of the points A,B,C in the layer, they will form the Matrix A, read the coords of the matching points K,L,M they will form Matrix B.
Outside Inkscape, using matrix calculator online, I do :
Mat A > inverse(Mat A), (inverse(Mat A)) * Mat B, and finaly transpose the result.
I get the coefficients needed to make the transformation using Object>transform>Matrix !

in the sample :
A= 856.862,63.833,1
B= 977.619,119.044,1
C= 1005.342,40.340,1
K= 157.152,250.870,1
L= 109.273,353.902,1
M= 168.368,375.494,1

Mat A = (A,B,C)
Mat B = (K,L,M)

Coefficients to use are :
-0.046, -0.767, 245.370
0.843, 0.023, -472.821

Everything works well and fine...
Perhaps this could be used by other to do an extension ??
Regards,
Didier

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Revision history for this message
 Houbrechts Didier (tetrao) wrote on 2017-12-06: #1
Revision history for this message
 Houbrechts Didier (tetrao) wrote on 2017-12-06: #2

and the result after move (matching the three points ...

 description: updated
Revision history for this message
 grey tomorrow (gtomorrow) wrote on 2020-09-11: #3

Hi - thanks for reporting this bug, I've manually migrated it to Inkscape's new bug tracker on GitLab, and closed it here.

Please feel free to file new bugs about the issues you're seeing at
https://inkscape.org/report .

Moved to: https://gitlab.com/inkscape/inbox/issues/3588
Closed by: https://gitlab.com/greytomorrow

 Changed in inkscape: status: New → Invalid