Since a signed 2-bit integer can only represent -1, 0, or 1, it is tempting to simply to convert it directly to a float. This maps it onto the correct range of [-1.0, 1.0]. However, it gives different values compared to the usual equation:
(2.0 * 1.0 + 1.0) * (1.0 / 3.0) = +1.0 (same) (2.0 * 0.0 + 1.0) * (1.0 / 3.0) = +0.33333333... (different) (2.0 * -1.0 + 1.0) * (1.0 / 3.0) = -0.33333333... (different) According to the GL_ARB_vertex_type_2_10_10_10_rev extension, signed normalization is performed using equation 2.2 from the GL 3.2 specification, which is: f = (2c + 1)/(2^b - 1). (2.2) Comments below that equation state: "In general, this representation is used for signed normalized fixed-point parameters in GL commands, such as vertex attribute values." Which is what we're doing here. The 3.2 specification goes on to declare an alternate formula: f = max{c/(2^(b-1) - 1), -1.0} (2.3) which is closer to the existing code, and maps the end points to exactly -1.0 and 1.0. Comments below the equation state: "In general, this representation is used for signed normalized fixed-point texture or framebuffer values." Which is *not* what we're doing here. It then states: "Everywhere that signed normalized fixed-point values are converted, the equation used is specified." This is the real clincher: the extension explicitly specifies that we must use equation 2.2, not 2.3. So we need to do (2x + 1) / 3. This matches the behavior expected by oglconform's packed-vertex test. Signed-off-by: Kenneth Graunke <kenn...@whitecape.org> --- src/mesa/vbo/vbo_attrib_tmp.h | 2 +- 1 file changed, 1 insertion(+), 1 deletion(-) diff --git a/src/mesa/vbo/vbo_attrib_tmp.h b/src/mesa/vbo/vbo_attrib_tmp.h index 8ad7354..bd00e0f 100644 --- a/src/mesa/vbo/vbo_attrib_tmp.h +++ b/src/mesa/vbo/vbo_attrib_tmp.h @@ -116,7 +116,7 @@ static inline float conv_i2_to_norm_float(int i2) { struct attr_bits_2 val; val.x = i2; - return (float)val.x; + return (2.0F * (float)val.x + 1.0F) * (1.0F / 3.0F); } #define ATTRI10_1( A, I10 ) ATTR( A, 1, conv_i10_to_i((I10) & 0x3ff), 0, 0, 1 ) -- 1.7.12.2 _______________________________________________ mesa-dev mailing list mesa-dev@lists.freedesktop.org http://lists.freedesktop.org/mailman/listinfo/mesa-dev