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Bug 1304 - Axis-Aligned Scaling incorrect on Projective Transforms
Summary: Axis-Aligned Scaling incorrect on Projective Transforms
Alias: None
Product: Eigen
Classification: Unclassified
Component: Geometry (show other bugs)
Version: unspecified
Hardware: All All
: Normal Wrong Result
Assignee: Nobody
Depends on:
Reported: 2016-09-23 01:01 UTC by Stephen Kiazyk
Modified: 2019-12-04 16:17 UTC (History)
4 users (show)

Compile this file with Eigen on the include search path. Then run on the command line (996 bytes, text/x-c++src)
2016-09-23 01:01 UTC, Stephen Kiazyk
no flags Details

Description Stephen Kiazyk 2016-09-23 01:01:37 UTC
Created attachment 736 [details]
Compile this file with Eigen on the include search path. Then run on the command line

When applying an axis-aligned scale (using `DiagonalMatrix`) to a `Transform` object whose `Mode` template argument is set to `Projective`, and whose lowest row elements are non-zero, the result is incorrect. That is, the scaling will not be applied to the elements in the final row. I'm assuming this is just a mis-assumption about the matrix, since this is technically correct for an `Affine` transform type.

Attached is an example demonstrating this. If I apply the `DiagonalMatrix` directly to a particular `Transform`, I get a different result than if I convert the `DiagonalMatrix` to a `Transform`, and then apply it to the same transform as before, I get different results.

I don't think this should be terribly difficult to solve, and if I have the time, I might look into fixing it, I'm just unfamiliar with the internals of Eigen, so I'm sure someone with a little more familiarity could solve it in a few minutes.

This file should compile with the following (with /usr/local/include/eigen3 replaced with whatever your install dir for Eigen is):

g++ -I/usr/local/include/eigen3 eigen_diagonal_affine_scale_problem.cpp -o eigen_diagonal_affine_scale_problem

And run with:


I have only tested this on Linux so far, however I doubt it wouldn't show up on any other platform.

- Stephen Kiazyk
Comment 1 Gael Guennebaud 2016-09-23 11:45:27 UTC
Thank you for catching that one. Fixed:
Summary:     Bug 1304: fix Projective * scaling and Projective *= scaling

Backport to 3.2:
Comment 2 Nobody 2019-12-04 16:17:46 UTC
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