New Transform type: Homography #11949
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Add new transform type, Homography. Add functions to compute homography from a list of GCPs. Add functions to serialize and deserialize a homography Automatically select homography transfrom when there are 4 or 5 GCPs present.
alg/gdal_homography.cpp
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| GDALMatrix LtL(9, 9); |
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are there references giving context for all the below maths that could be included as code comments ? / how did you derive that ?
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I've added a link to https://www.cs.unc.edu/~ronisen/teaching/fall_2023/pdf_slides/lecture9_transformation.pdf, and changed LtL to AtA to match the notation in that document. Is there a preferred alternative to this kind of link? I can't guarantee how long the link will be available.
alg/gdal_homography.cpp
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| return FALSE; | ||
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| // "Hour-glass" like shape of GCPs. Cf https://github.com/OSGeo/gdal/issues/11618 |
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I don't understand that comment. Hour-glass like shape is one possibility, not necesarily all the cases
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Hopefully the new comments clarify things.
| } | ||
| double cross12 = x[1] * y[2] - x[2] * y[1]; | ||
| double cross23 = x[2] * y[3] - x[3] * y[2]; | ||
| if (cross12 * cross23 <= 0.0) |
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what does this check do ? Should we emit an error when it fails ?
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This check verifies that the normalized transform maps the unit square to a convex quadrilateral.
Regarding emitting an error, I followed the precedent of GDALGCPsToGeoTransform(), which returns FALSE without emitting an error in all cases that fail to compute a valid GeoTransform. I can change it to emit errors in those cases if needed.
| /* Compose the resulting transformation with the normalization */ | ||
| /* homographies. */ | ||
| /* -------------------------------------------------------------------- */ | ||
| double h1p2[9] = {0.0}; |
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Following the precedent set by gt1p2 in GDALGCPsToGeoTransform() (gt = geotransform, h = homography).
This is an intermediate homography, which maps between pixels and normalized geo coordinates.
Co-authored-by: Even Rouault <even.rouault@spatialys.com>
Co-authored-by: Even Rouault <even.rouault@spatialys.com>
Co-authored-by: Even Rouault <even.rouault@spatialys.com>
Co-authored-by: Even Rouault <even.rouault@spatialys.com>
Co-authored-by: Even Rouault <even.rouault@spatialys.com>
What does this PR do?
Adds new transform type, Homography.
Adds functions to compute homography from a list of GCPs.
Adds functions to serialize and deserialize a homography
Automatically selects homography transform when there are 4 or 5 GCPs present. Homography provides an exact fit for 4 GCPs, and a better fit for 5 GCPs than the existing method (affine transformation).
What are related issues/pull requests?
#11940
Tasklist
- [ ] AI (Copilot or something similar) supported my development of this PR- [ ] Updated Python API documentation (swig/include/python/docs/)Environment
Provide environment details, if relevant: