DOC: Include description of pre-processing transforms

Documentation/conf.py

@@ -27,6 +27,7 @@
     "sphinx_rtd_theme",
     "sphinxcontrib.autoprogram",
     "sphinxcontrib.bibtex",
+    "sphinxcontrib.mermaid",
 ]
 bibtex_bibfiles = ["refs.bib"]
 myst_enable_extensions = [

Documentation/index.md

@@ -14,6 +14,7 @@ getting-started.md
 get-help.md
 user-interface.md
 shortcuts.md
+transforms.md
 tutorials/index.md
 adv-topics/index.md
 file-specifications/index.md

Documentation/requirements.txt

@@ -6,3 +6,4 @@ sphinx-notfound-page
 sphinx-rtd-theme == 1.0.0rc1
 sphinxcontrib-autoprogram
 sphinxcontrib-bibtex
+sphinxcontrib-mermaid

Documentation/transforms.md

@@ -0,0 +1,166 @@
+# Transforms
+
+## Background
+
+:::{figure-md} Slicer-coordinate-systems
+:align: center
+
+![Coordinate systems](https://github.com/Slicer/Slicer/releases/download/docs-resources/coordinate_systems.png)
+
+World coordinate system (left), Anatomical coordinate system (middle), Image coordinate system (right) Image source: [Slicer Coordinate Systems documentation](https://slicer.readthedocs.io/en/latest/user_guide/coordinate_systems.html)
+:::
+
+In 3D Slicer, medical image data is processed using the Right-Anterior-Superior (RAS) coordinate
+system by default. However, to maintain compatibility with other medical imaging software, Slicer
+assumes that files are stored in the Left-Posterior-Superior (LPS) coordinate system unless
+otherwise specified.
+
+```{image} https://github.com/BrownBiomechanics/Autoscoper/releases/download/docs-resources/transforms_XYZ_axes.png
+:align: center
+:alt: XYZ Axes
+```
+
+When reading or writing files, Slicer may need to flip the sign of the first
+two coordinate axes to convert between RAS and LPS. The following transformation
+matrix converts between RAS and LPS coordinates:
+
+```{image} https://github.com/BrownBiomechanics/Autoscoper/releases/download/docs-resources/transforms_LPS_to_RAS_coords.png
+:align: center
+:alt: LPS to RAS Coordinate Transformation
+```
+
+For more details on the coordinate systems used in 3D Slicer, refer to the [Slicer documentation](https://slicer.readthedocs.io/en/latest/user_guide/coordinate_systems.html).
+
+### Spatial referencing
+
+Spatial referencing data is used to encode voxel to world unit resolution (also referred to as pixel/voxel spacing), origin,
+and orientation. In DICOM format, the spatial referencing can be retrieved from the DICOM header meta data. In MATLAB,
+the [`imref3d`](https://www.mathworks.com/help/images/ref/imref3d.html) function can be constructed from the DICOM meta data to store the intrinsic spatial relationship.
+
+Transforming between image space and world space results in proportional changes in the image volume:
+
+```{image} https://github.com/BrownBiomechanics/Autoscoper/releases/download/docs-resources/transforms_Image_to_world_space.png
+:align: center
+:alt: Image space to World Space
+```
+
+The matrix shown above can be used to convert from image space to world space, where `P_{x,y,z}` represents pixel spacing along
+each axis, and `O_{x,y,z}` is the origin for each respective dimension.
+
+When importing DICOM data into Slicer, a spatial referencing transform is automatically applied to a CT image volume based on the
+header metadata in the DICOM header. Note the RAS anatomical orientation and +X, +Y, +Z (red/green/blue) axes indicators.
+
+```{image} https://github.com/BrownBiomechanics/Autoscoper/releases/download/docs-resources/transforms_Slicer_DICOM_data.png
+:align: center
+:alt: DICOM Data Loaded into Slicer
+```
+
+Spatial referencing information of a volume in Slicer can be viewed in the Volume Information drop down of the
+Volumes module.
+
+```{image} https://github.com/BrownBiomechanics/Autoscoper/releases/download/docs-resources/transforms_Volume_spatial_info.png
+:align: center
+:alt: Spacial Referencing Information of a Volume
+```
+
+## Transforms in SlicerAutoscoperM
+
+SlicerAutoscoperM enables tracking of rigid bodies in the 'World' space. When tracking multiple
+rigid bodies, their relative motions can be computed with respect to a reference body.
+
+In the AutoscoperM Slicer module, under the Pre-processing tab, rigid bodies of interest can
+be automatically segmented from a CT DICOM volume. Alternatively, previously generated segmentations
+can be loaded. These segmentations are used to generate partial volumes, where the density data
+within the bounds of the segmented model is isolated and saved as a TIFF stack. In Autoscoper, this
+TIFF stack is imported in a specific orientation, referred to as Autoscoper (AUT) space. The data from the
+partial volume is projected onto the target image plane (overlaid with the radiograph for each frame)
+as a Digitally Reconstructed Radiograph (DRR).
+
+Like CT DICOM volumes, TIFF stacks are initially in image space and require a
+spatial referencing transform to describe their world location, spacing, and
+orientation. In Autoscoper, the voxel resolution of TIFF volumes follows a specified axes.
+
+```{image} https://github.com/BrownBiomechanics/Autoscoper/releases/download/docs-resources/transforms_Model_and_its_TIFF_pv.png
+:align: center
+:alt: Segmented Model and Corresponding TIFF Stack
+```
+
+To facilitate post-processing of output transforms from Autoscoper, several folders
+are populated at the time of Partial Volume Generation in SlicerAutoscoperM. The
+following structure outlines the information in the output directory:
+
+```
+Output Directory
+│
+├── Models
+│   └── AUT{bone}.stl
+│
+├── Tracking
+│   └── {bone}.tra
+│
+├── Transforms
+│   ├── {bone}.tfm
+│   ├── {bone}_DICOM2AUT.tfm
+│   ├── {bone}_PVOL2AUT.tfm
+│   ├── {bone}_scale.tfm
+│   └── {bone}_t.tfm
+│
+└── Volumes
+    └── {bone}.tif
+```
+
+* **Models:**
+  * `AUT{bone}.stl`: Mesh file of the volume segmentation placed in Autoscoper (AUT) space.
+* **Tracking:**
+  * `{bone}.tra`: Equivalent to `{bone}_DICOM2AUT.tfm` but formatted for Autoscoper compatibility.
+* **Transforms:**
+  * `{bone}.tfm`: Non-rigid transform that translates and scales the `{bone}.tif` volume to its spatial location within the segmented CT-DICOM.
+  * `{bone}_DICOM2AUT.tfm`: Transformation from DICOM space into Autoscoper (AUT) space.
+  * `{bone}_PVOL2AUT.tfm`: Transformation from world space into Autoscoper (AUT) space.
+  * `{bone}_scale.tfm`: Scaling matrix that converts the volume from image space to world space.
+  * `{bone}_t.tfm`: Translation matrix moving between the world origin and the location of the partial volume within the segmented CT-DICOM.
+* **Volumes:**
+  * `{bone}.tif`: Non-spatially transformed volumetric data segmented from CT-DICOM.
+
+
+:::{figure-md} Tfms-to-dicom-space
+:align: center
+
+![Transforms of TIFF Stack to DICOM Space](https://github.com/BrownBiomechanics/Autoscoper/releases/download/docs-resources/transforms_Tfms_to_DICOM_space.png)
+
+Transforms to DICOM space: `{bone}.tfm` (pink arrow), `{bone}_t.tfm` (blue arrow)
+:::
+
+
+:::{figure-md} Tfms-to-aut-space
+:align: center
+
+![Transforms of PV to AUT Space](https://github.com/BrownBiomechanics/Autoscoper/releases/download/docs-resources/transforms_Tfms_to_AUT_space.png)
+
+Transforms to Autoscoper space: `{bone}_DICOM2AUT.tfm` (orange arrow), `{bone}_t.tfm` (blue arrow) and `{bone}_PVOL2AUT.tfm` (gray arrow)
+:::
+
+
+:::{mermaid}
+:align: center
+
+flowchart TD
+    subgraph image_space["Image Space"]
+      tiff["TIFF"]
+    end
+    subgraph dicom_space["DICOM Space"]
+      transformed_pvol["Spatially located PV"]
+    end
+    subgraph world_space["World Space"]
+      world_origin["World origin"]
+      partial_volume["Partial Volume (PVOL)"]
+    end
+    subgraph aut_space["Autoscoper Space (AUT)"]
+      model["Model"]
+    end
+    world_space -- "{bone}_t.tfm" --> dicom_space
+    image_space -- "{bone}.tfm" --> dicom_space
+    image_space -- "{bone}_scale.tfm" --> world_space
+    dicom_space -- "{bone}_DICOM2AUT.tfm" --> aut_space
+    world_space -- "{bone}_PVOL2AUT.tfm" --> aut_space
+:::