pick matrix helper (aka gluPickMatrix)

CREDITS

@@ -78,3 +78,7 @@ https://stackoverflow.com/a/57793352/2676533
 16. ARM NEON Div
 
 http://github.com/microsoft/DirectXMath
+
+17. Pick Matrix
+
+glu project -> project.c

include/cglm/call/project.h

@@ -25,6 +25,10 @@ CGLM_EXPORT
 void
 glmc_project(vec3 pos, mat4 m, vec4 vp, vec3 dest);
 
+CGLM_EXPORT
+void
+glmc_pickmatrix(vec2 center, vec2 size, vec4 vp, mat4 dest);
+
 #ifdef __cplusplus
 }
 #endif

include/cglm/project.h

@@ -114,4 +114,37 @@ glm_project(vec3 pos, mat4 m, vec4 vp, vec3 dest) {
 #endif
 }
 
+/*!
+ * @brief define a picking region
+ *
+ * @param[in]  center   center [x, y] of a picking region in window coordinates
+ * @param[in]  size     size [width, height] of the picking region in window coordinates
+ * @param[in]  vp       viewport as [x, y, width, height]
+ * @param[out] dest     projected coordinates
+ */
+CGLM_INLINE
+void
+glm_pickmatrix(vec3 center, vec2 size, vec4 vp, mat4 dest) {
+  mat4 res;
+  vec3 v;
+
+  if (size[0] <= 0.0f || size[1] <= 0.0f)
+    return;
+  
+  /* Translate and scale the picked region to the entire window */
+  v[0] = (vp[2] - 2.0f * (center[0] - vp[0])) / size[0];
+  v[1] = (vp[3] - 2.0f * (center[1] - vp[1])) / size[1];
+  v[2] = 0.0f;
+
+  glm_translate_make(res, v);
+  
+  v[0] = vp[2] / size[0];
+  v[1] = vp[3] / size[1];
+  v[2] = 1.0f;
+
+  glm_scale(res, v);
+
+  glm_mat4_copy(res, dest);
+}
+
 #endif /* cglm_project_h */

include/cglm/struct/project.h

@@ -101,4 +101,20 @@ glms_project(vec3s pos, mat4s m, vec4s vp) {
   return r;
 }
 
+/*!
+ * @brief define a picking region
+ *
+ * @param[in]  center   center [x, y] of a picking region in window coordinates
+ * @param[in]  size     size [width, height] of the picking region in window coordinates
+ * @param[in]  vp       viewport as [x, y, width, height]
+ * @returns projected coordinates
+ */
+CGLM_INLINE
+mat4s
+glms_pickmatrix(vec3s center, vec2s size, vec4s vp) {
+  mat4s res;
+  glm_pickmatrix(center.raw, size.raw, vp.raw, res.raw);
+  return res;
+}
+
 #endif /* cglms_projects_h */

src/project.c

@@ -25,3 +25,9 @@ void
 glmc_project(vec3 pos, mat4 m, vec4 vp, vec3 dest) {
   glm_project(pos, m, vp, dest);
 }
+
+CGLM_EXPORT
+void
+glmc_pickmatrix(vec2 center, vec2 size, vec4 vp, mat4 dest) {
+  glm_pickmatrix(center, size, vp, dest);
+}