Fix computation of camera distance

Compute the value of the camera distance as exactly half the xx
component of the projection matrix. The heuristically derived
value for 60 degrees was off by about 0.016%, causing noticeable
blurring, and other field of view angles which didn't have the
heuristic adjustment off by much more.

Signed-off-by: Emmanuele Bassi <ebassi@linux.intel.com>

diff --git a/clutter/cogl/gl/cogl.c b/clutter/cogl/gl/cogl.c
index aa0ec78..78b2a90 100644 (file)
--- a/clutter/cogl/gl/cogl.c
+++ b/clutter/cogl/gl/cogl.c
@@ -723,37 +723,55 @@ cogl_setup_viewport (guint        width,
                     float z_far)
 {
   GLfloat z_camera;
+  GLfloat projection_matrix[16];
 
   GE( glViewport (0, 0, width, height) );
 
   cogl_perspective (fovy, aspect, z_near, z_far);
 
-  GE( glLoadIdentity () );
-
   /*
-   * camera distance from screen, 0.5 * tan (FOV)
+   * In theory, we can compute the camera distance from screen as:
+   *
+   *   0.5 * tan (FOV)
+   *
+   * However, it's better to compute the z_camera from our projection
+   * matrix so that we get a 1:1 mapping at the screen distance. Consider
+   * the upper-left corner of the screen. It has object coordinates
+   * (0,0,0), so by the transform below, ends up with eye coordinate
+   *
+   *   x_eye = x_object / width - 0.5 = - 0.5
+   *   y_eye = (height - y_object) / width - 0.5 = 0.5
+   *   z_eye = z_object / width - z_camera = - z_camera
+   *
+   * From cogl_perspective(), we know that the projection matrix has
+   * the form:
+   *
+   *  (x, 0,  0, 0)
+   *  (0, y,  0, 0)
+   *  (0, 0,  c, d)
+   *  (0, 0, -1, 0)
    *
-   * We have been having some problems with this; the theoretically correct
-   * value of 0.866025404f for the default 60 deg fovy angle happens to be
-   * touch to small in reality, which on full-screen stage with an actor of
-   * the same size results in about 1px on the left and top edges of the
-   * actor being offscreen. Perhaps more significantly, it also causes
-   * hinting artifacts when rendering text.
+   * Applied to the above, we get clip coordinates of
    *
-   * So for the default 60 deg angle we worked out that the value of 0.869
-   * is giving correct stretch and no noticeable artifacts on text. Seems
-   * good on all drivers too.
+   *  x_clip = x * (- 0.5)
+   *  y_clip = y * 0.5
+   *  w_clip = - 1 * (- z_camera) = z_camera
+   *
+   * Dividing through by w to get normalized device coordinates, we
+   * have, x_nd = x * 0.5 / z_camera, y_nd = - y * 0.5 / z_camera.
+   * The upper left corner of the screen has normalized device coordinates,
+   * (-1, 1), so to have the correct 1:1 mapping, we have to have:
+   *
+   *   z_camera = 0.5 * x = 0.5 * y
+   *
+   * If x != y, then we have a non-uniform aspect ration, and a 1:1 mapping
+   * doesn't make sense.
    */
-#define DEFAULT_Z_CAMERA 0.869f
-  z_camera = DEFAULT_Z_CAMERA;
 
+  cogl_get_projection_matrix (projection_matrix);
+  z_camera = 0.5 * projection_matrix[0];
 
-  if (fovy != 60.0)
-  {
-    float fovy_rad = (fovy * G_PI) / 180;
-
-    z_camera = ((sinf (fovy_rad) / cosf (fovy_rad)) / 2);
-  }
+  GE( glLoadIdentity () );
 
   GE( glTranslatef (-0.5f, -0.5f, -z_camera) );
   GE( glScalef ( 1.0f / width,