cam: extend frustum's far distance helper (#71)

* this will help to implement zoom easily

docs/source/cam.rst

@@ -36,6 +36,7 @@ Functions:
 #. :c:func:`glm_ortho_default`
 #. :c:func:`glm_ortho_default_s`
 #. :c:func:`glm_perspective`
+#. :c:func:`glm_persp_move_far`
 #. :c:func:`glm_perspective_default`
 #. :c:func:`glm_perspective_resize`
 #. :c:func:`glm_lookat`
@@ -145,6 +146,16 @@ Functions documentation
       | *[in]*  **farVal**  far clipping planes
       | *[out]* **dest**    result matrix
 
+.. c:function:: void  glm_persp_move_far(mat4 proj, float deltaFar)
+
+    | extend perspective projection matrix's far distance
+
+    | this function does not guarantee far >= near, be aware of that!
+
+    Parameters:
+      | *[in, out]*  **proj**      projection matrix to extend
+      | *[in]*       **deltaFar**  distance from existing far (negative to shink)
+
 .. c:function:: void glm_perspective_default(float aspect, mat4 dest)
 
      | set up perspective projection matrix with default near/far

include/cglm/call/cam.h

@@ -61,6 +61,10 @@ glmc_perspective(float fovy,
                  float farVal,
                  mat4 dest);
 
+CGLM_EXPORT
+void
+glmc_persp_move_far(mat4 proj, float deltaFar);
+
 CGLM_EXPORT
 void
 glmc_perspective_default(float aspect, mat4 dest);

include/cglm/cam.h

@@ -271,6 +271,30 @@ glm_perspective(float fovy,
   dest[3][2] = 2.0f * nearVal * farVal * fn;
 }
 
+/*!
+ * @brief extend perspective projection matrix's far distance
+ *
+ * this function does not guarantee far >= near, be aware of that!
+ *
+ * @param[in, out] proj      projection matrix to extend
+ * @param[in]      deltaFar  distance from existing far (negative to shink)
+ */
+CGLM_INLINE
+void
+glm_persp_move_far(mat4 proj, float deltaFar) {
+  float fn, farVal, nearVal, p22, p32;
+
+  p22        = proj[2][2];
+  p32        = proj[3][2];
+
+  nearVal    = p32 / (p22 - 1.0f);
+  farVal     = p32 / (p22 + 1.0f) + deltaFar;
+  fn         = 1.0f / (nearVal - farVal);
+
+  proj[2][2] = (nearVal + farVal) * fn;
+  proj[3][2] = 2.0f * nearVal * farVal * fn;
+}
+
 /*!
  * @brief set up perspective projection matrix with default near/far
  *        and angle values

src/cam.c

@@ -88,6 +88,12 @@ glmc_perspective(float fovy,
                   dest);
 }
 
+CGLM_EXPORT
+void
+glmc_persp_move_far(mat4 proj, float deltaFar) {
+  glm_persp_move_far(proj, deltaFar);
+}
+
 CGLM_EXPORT
 void
 glmc_perspective_default(float aspect, mat4 dest) {