add docs for project/unproject

docs/source/api.rst

@@ -41,6 +41,7 @@ Follow the :doc:`build` documentation for this
    vec4-ext
    color
    plane
+   project
    util
    io
    call

docs/source/project.rst

@@ -0,0 +1,102 @@
+.. default-domain:: C
+
+Project / UnProject
+================================================================================
+
+Header: cglm/project.h
+
+Viewport is required as *vec4* **[X, Y, Width, Height]** but this doesn't mean
+that you should store it as **vec4**. You can convert your data representation
+to vec4 before passing it to related functions.
+
+Table of contents (click to go):
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+Functions:
+
+1. :c:func:`glm_unprojecti`
+#. :c:func:`glm_unproject`
+#. :c:func:`glm_project`
+
+Functions documentation
+~~~~~~~~~~~~~~~~~~~~~~~
+
+.. c:function:: void  glm_unprojecti(vec3 pos, mat4 invMat, vec4 vp, vec3 dest)
+
+    | maps the specified viewport coordinates into specified space [1]
+      the matrix should contain projection matrix.
+
+    if you don't have ( and don't want to have ) an inverse matrix then use
+    glm_unproject version. You may use existing inverse of matrix in somewhere
+    else, this is why glm_unprojecti exists to save save inversion cost
+
+    [1] space:
+      - if m = invProj:     View Space
+      - if m = invViewProj: World Space
+      - if m = invMVP:      Object Space
+
+    You probably want to map the coordinates into object space
+    so use invMVP as m
+
+    Computing viewProj:
+
+    .. code-block:: c
+
+       glm_mat4_mul(proj, view, viewProj);
+       glm_mat4_mul(viewProj, model, MVP);
+       glm_mat4_inv(viewProj, invMVP);
+
+    Parameters:
+      | *[in]*  **pos**     point/position in viewport coordinates
+      | *[in]*  **invMat**  matrix (see brief)
+      | *[in]*  **vp**      viewport as [x, y, width, height]
+      | *[out]* **dest**    unprojected coordinates
+
+.. c:function:: void  glm_unproject(vec3 pos, mat4 m, vec4 vp, vec3 dest)
+
+    | maps the specified viewport coordinates into specified space [1]
+      the matrix should contain projection matrix.
+
+    this is same as glm_unprojecti except this function get inverse matrix for
+    you.
+
+    [1] space:
+      - if m = proj:     View Space
+      - if m = viewProj: World Space
+      - if m = MVP:      Object Space
+
+    You probably want to map the coordinates into object space so use MVP as m
+
+    Computing viewProj and MVP:
+
+    .. code-block:: c
+
+       glm_mat4_mul(proj, view, viewProj);
+       glm_mat4_mul(viewProj, model, MVP);
+
+    Parameters:
+      | *[in]*  **pos**  point/position in viewport coordinates
+      | *[in]*  **m**    matrix (see brief)
+      | *[in]*  **vp**   viewport as [x, y, width, height]
+      | *[out]* **dest** unprojected coordinates
+
+.. c:function:: void  glm_project(vec3 pos, mat4 m, vec4 vp, vec3 dest)
+
+    | map object coordinates to window coordinates
+
+    Computing MVP:
+
+    .. code-block:: c
+
+       glm_mat4_mul(proj, view, viewProj);
+       glm_mat4_mul(viewProj, model, MVP);
+
+    this could be useful for gettng a bbox which fits with view frustum and
+    object bounding boxes. In this case you crop view frustum box with objects
+    box
+
+    Parameters:
+      | *[in]*  **pos**      object coordinates
+      | *[in]*  **m**        MVP matrix
+      | *[in]*  **vp**       viewport as [x, y, width, height]
+      | *[out]* **dest**     projected coordinates