perspective sizes

include/cglm/cam.h

@@ -478,6 +478,32 @@ glm_persp_aspect(mat4 proj) {
   return proj[1][1] / proj[0][0];
 }
 
+/*!
+ * @brief returns aspect ratio of perspective projection
+ *
+ * if you don't have fovy then use glm_persp_fovy(proj) to get it
+ * or pass directly: glm_persp_sizes(proj, glm_persp_fovy(proj), sizes);
+ *
+ * @param[in]  proj perspective projection matrix
+ * @param[in]  fovy fovy (see brief)
+ * @param[out] dest sizes order: [Wnear, Hnear, Wfar, Hfar]
+ */
+CGLM_INLINE
+void
+glm_persp_sizes(mat4 proj, float fovy, vec4 dest) {
+  float t, a, nearVal, farVal;
+
+  t = 2.0f * tanf(fovy * 0.5f);
+  a = glm_persp_aspect(proj);
+
+  glm_persp_decomp_z(proj, &nearVal, &farVal);
+
+  dest[1]  = t * nearVal;
+  dest[3]  = t * farVal;
+  dest[0]  = a * dest[1];
+  dest[2]  = a * dest[3];
+}
+
 /*!
  * @brief extracts view frustum planes
  *

test/src/test_cam.c

@@ -9,7 +9,8 @@
 
 void
 test_camera_decomp(void **state) {
-  mat4  proj;
+  mat4  proj, proj2;
+  vec4  sizes;
   float aspect, fovy, nearVal, farVal;
 
   aspect  = 0.782f;
@@ -21,5 +22,17 @@ test_camera_decomp(void **state) {
   assert_true(fabsf(aspect  - glm_persp_aspect(proj)) < FLT_EPSILON);
   assert_true(fabsf(fovy    - glm_persp_fovy(proj))   < FLT_EPSILON);
   assert_true(fabsf(49.984f - glm_deg(glm_persp_fovy(proj))) < FLT_EPSILON);
+
+  glm_persp_sizes(proj, fovy, sizes);
+
+  glm_frustum(-sizes[0] * 0.5,
+               sizes[0] * 0.5,
+              -sizes[1] * 0.5,
+               sizes[1] * 0.5,
+               nearVal,
+               farVal,
+               proj2);
+
+  test_assert_mat4_eq(proj, proj2);
 }