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Fix computation of camera distance

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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>
This commit is contained in:
Owen W. Taylor 2009-01-22 14:17:16 +00:00 committed by Emmanuele Bassi
parent dcbb0833c1
commit 66331b6863
1 changed files with 38 additions and 20 deletions
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58
gl/cogl.c
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@ -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:
*
* 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.
* 0.5 * tan (FOV)
*
* 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.
* 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)
*
* Applied to the above, we get clip coordinates of
*
* 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,