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authorKenneth Graunke <[email protected]>2012-10-12 12:46:44 -0700
committerKenneth Graunke <[email protected]>2012-11-21 20:32:54 -0800
commitbf75a1f09211a9ca90b2ea3366b3331919793f9d (patch)
tree50a4433e77f86f09081907d2254b65bc89aefccc /src/mesa/drivers/x11
parente9967aba61a7cc62ce13ab691d14bb5a26a86896 (diff)
mesa/vbo: Fix scaling issue in 2-bit signed normalized packing.
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, and is correct for desktop GL (pre-4.2). It's not correct for ES 3.0, but a future patch will correct that. Signed-off-by: Kenneth Graunke <[email protected]> Tested-by: Marek Olšák <[email protected]>
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