refactor(docs): consolidate features.rst content into index.rst for b…

…etter organization and readability

docs/index.rst

@@ -29,8 +29,106 @@ clear and easy-to-use APIs.
    :caption: Docs
 
    Home <self>
-   features
    coordinates
    installation
    contributing
    api
+
+
+What can you do with CamTools?
+------------------------------
+
+1. Plot cameras
+^^^^^^^^^^^^^^^
+
+Useful for debugging 3D reconstruction and NeRFs!
+
+.. code-block:: python
+
+   import camtools as ct
+   import open3d as o3d
+   cameras = ct.camera.create_camera_frustums(Ks, Ts)
+   o3d.visualization.draw_geometries([cameras])
+
+.. raw:: html
+
+   <p align="center">
+      <img src="https://raw.githubusercontent.com/yxlao/camtools/main/camtools/assets/camera_frames.png" width="360" />
+   </p>
+
+2. Convert camera parameters
+^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+.. code-block:: python
+
+   pose = ct.convert.T_to_pose(T)     # Convert T to pose
+   T    = ct.convert.pose_to_T(pose)  # Convert pose to T
+   R, t = ct.convert.T_to_R_t(T)      # Convert T to R and t
+   C    = ct.convert.pose_to_C(pose)  # Convert pose to camera center
+   K, T = ct.convert.P_to_K_T(P)      # Decompose projection matrix P to K and T
+                                      # And more...
+
+3. Projection and ray casting
+^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+.. code-block:: python
+
+   # Project 3D points to pixels.
+   pixels = ct.project.points_to_pixel(points, K, T)
+
+   # Back-project depth image to 3D points.
+   points = ct.project.im_depth_to_points(im_depth, K, T)
+
+   # Ray cast a triangle mesh to depth image given the camera parameters.
+   im_depth = ct.raycast.mesh_to_im_depth(mesh, K, T, height, width)
+
+   # And more...
+
+4. Image and depth I/O
+^^^^^^^^^^^^^^^^^^^^^^
+
+Strict type checks and range checks are enforced. The image and depth I/O
+APIs are specifically designed to solve the following pain points:
+
+- Is my image of type ``float32`` or ``uint8``?
+- Does it have range ``[0, 1]`` or ``[0, 255]``?
+- Is it RGB or BGR?
+- Does my image have an alpha channel?
+- When saving depth image as integer-based ``.png``, is it correctly scaled?
+
+.. code-block:: python
+
+   ct.io.imread()
+   ct.io.imwrite()
+   ct.io.imread_detph()
+   ct.io.imwrite_depth()
+
+5. Command-line tools
+^^^^^^^^^^^^^^^^^^^^^
+
+The ``ct`` command runs in terminal:
+
+.. code-block:: bash
+
+   # Crop image boarders.
+   ct crop-boarders *.png --pad_pixel 10 --skip_cropped --same_crop
+
+   # Draw synchronized bounding boxes interactively.
+   ct draw-bboxes path/to/a.png path/to/b.png
+
+   # For more command-line tools.
+   ct --help
+
+.. raw:: html
+
+   <p align="center">
+      <img src="https://user-images.githubusercontent.com/1501945/241416210-e11ff3bf-22e6-46c0-8ba0-d177a0015323.png" width="400" />
+   </p>
+
+6. And more
+^^^^^^^^^^^
+
+- Solve line intersections
+- COLMAP tools
+- Points normalization
+- And more...