change Camera2D behavior (#945)

examples/core/core_2d_camera.c

@@ -42,7 +42,7 @@ int main(void)
 
     Camera2D camera = { 0 };
     camera.target = (Vector2){ player.x + 20, player.y + 20 };
-    camera.offset = (Vector2){ 0, 0 };
+    camera.offset = (Vector2){ screenWidth / 2, screenHeight / 2 };
     camera.rotation = 0.0f;
     camera.zoom = 1.0f;
 
@@ -57,12 +57,10 @@ int main(void)
         if (IsKeyDown(KEY_RIGHT))
         {
             player.x += 2;              // Player movement
-            camera.offset.x -= 2;       // Camera displacement with player movement
         }
         else if (IsKeyDown(KEY_LEFT))
         {
             player.x -= 2;              // Player movement
-            camera.offset.x += 2;       // Camera displacement with player movement
         }
 
         // Camera target follows player

src/core.c

@@ -1251,11 +1251,24 @@ void BeginMode2D(Camera2D camera)
     rlLoadIdentity();                   // Reset current matrix (MODELVIEW)
     rlMultMatrixf(MatrixToFloat(screenScaling)); // Apply screen scaling if required
 
-    // Camera rotation and scaling is always relative to target
+    //The camera in world-space is set by
+    //1. Move it to target
+    //2. Rotate by -rotation and scale by (1/zoom)
+    //      When setting higher scale, it's more intuitive for the world to become bigger (= camera become smaller),
+    //      not for the camera getting bigger, hence the invert. Same deal with rotation.
+    //3. Move it by (-offset);
+    //      Offset defines target transform relative to screen, but since we're effectively "moving" screen (camera)
+    //      we need to do it into opposite direction (inverse transform)
+    //
+    //Having camera transform in world-space, inverse of it gives the modelview transform.
+    //Since (A*B*C)' = C'*B'*A', the modelview is
+    //1. Move to offset
+    //2. Rotate and Scale
+    //3. Move by -target
     Matrix matOrigin = MatrixTranslate(-camera.target.x, -camera.target.y, 0.0f);
     Matrix matRotation = MatrixRotate((Vector3){ 0.0f, 0.0f, 1.0f }, camera.rotation*DEG2RAD);
     Matrix matScale = MatrixScale(camera.zoom, camera.zoom, 1.0f);
-    Matrix matTranslation = MatrixTranslate(camera.offset.x + camera.target.x, camera.offset.y + camera.target.y, 0.0f);
+    Matrix matTranslation = MatrixTranslate(camera.offset.x, camera.offset.y, 0.0f);
 
     Matrix matTransform = MatrixMultiply(MatrixMultiply(matOrigin, MatrixMultiply(matScale, matRotation)), matTranslation);