u_gen_mipmap: Use untampered cubemap texture coords when generating mipmaps.

It's not necessary to scale down cubemap texture coords when generating
mipmaps: we are doing a 2x minification therefore it's guaranteed that
the texture coords will always be at least 1 texel away of the edges.

Scaling down can actually be harmful, as it may cause artefacts when
generating mipmaps with nearest filtering.  Sample points will lie
exactly in the middle each 2x2 texels, so the scaling factor was causing
different texels to be take on each quadrant of the cube face.  This is
apparent with a 1x1 checkerboard pattern in the base mipmap level:
instead of next mipmap level receiving a constant color throughout the
face, it will have different colors for each quadrant of the face.

The behaviour for blits is left untouched for now, but the cubemap
texture coord scaling hack should be reconsidered eventually.

Reviewed-by: Brian Paul <[email protected]>

1 files changed, 9 insertions, 2 deletions

diff --git a/src/gallium/auxiliary/util/u_texture.c b/src/gallium/auxiliary/util/u_texture.c
index d97e57a7903..e865f8e73d0 100644
--- a/src/gallium/auxiliary/util/u_texture.c
+++ b/src/gallium/auxiliary/util/u_texture.c
@@ -42,7 +42,8 @@
 
 void util_map_texcoords2d_onto_cubemap(unsigned face,
                                        const float *in_st, unsigned in_stride,
-                                       float *out_str, unsigned out_stride)
+                                       float *out_str, unsigned out_stride,
+                                       boolean allow_scale)
 {
    int i;
    float rx, ry, rz;
@@ -52,8 +53,14 @@ void util_map_texcoords2d_onto_cubemap(unsigned face,
       /* Compute sc = +/-scale and tc = +/-scale.
        * Not +/-1 to avoid cube face selection ambiguity near the edges,
        * though that can still sometimes happen with this scale factor...
+       *
+       * XXX: Yep, there is no safe scale factor that will prevent sampling
+       * the neighbouring face when stretching out.  A more reliable solution
+       * would be to clamp (sc, tc) against +/- 1.0-1.0/mipsize, in the shader.
+       *
+       * Also, this is not necessary when minifying, or 1:1 blits.
        */
-      const float scale = 0.9999f;
+      const float scale = allow_scale ? 0.9999f : 1.0f;
       const float sc = (2 * in_st[0] - 1) * scale;
       const float tc = (2 * in_st[1] - 1) * scale;