Hi Lionel,
Indeed the value 8 here is questionable. I picked it because other
drivers advertise the same value e.g. in Mesa radeon returns 8 for gl
and vulkan or on Windows Intel's driver returns 8. But why 8? It's some
kind of mystery.
"If the implementation truely has floating point viewport bounds, it
may report a sufficiently high value to indicate this. "
8 seems to be a sufficiently high value (it seems if someone even checks
the value it's going like 'precision > 0' - it is used as a flag). But
still it's probably not good enough argument...
Floating point (IEEE 754) has 24 bits of significand precision, in other
way - 6 to 9 significant decimal digits. And drivers return 8, the only
8 in float-point is 8 exponent bits.
Unless someone knows why 8, there two paths:
* Left it to be 8 - be the same as other drivers
* Make 24 - to reflect 24 bits of significand precision of float
- Danil
On 18.06.18 17:27, Lionel Landwerlin wrote:
Hey Danylo,
Thanks for this patch.
I'm not really an expert here but my understanding is that it should
reflect the number of bits in fixed point precision.
We use 32bits floats in the packets sent to the hardware.
Quoting the spec :
"If the implementation truely has floating point viewport bounds, it
may report a sufficiently high value to indicate this. "
Maybe we should use something a bit bigger than 8?
Cheers,
-
Lionel
On 18/06/18 13:50, Danylo Piliaiev wrote:
We use floating-points for viewport bounds so VIEWPORT_SUBPIXEL_BITS
should reflect this.
Bugzilla:https://bugs.freedesktop.org/show_bug.cgi?id=105975
Signed-off-by: Danylo Piliaiev<[email protected]>
---
src/mesa/drivers/dri/i965/brw_context.c | 2 +-
1 file changed, 1 insertion(+), 1 deletion(-)
diff --git a/src/mesa/drivers/dri/i965/brw_context.c
b/src/mesa/drivers/dri/i965/brw_context.c
index 9ced230..eacf326 100644
--- a/src/mesa/drivers/dri/i965/brw_context.c
+++ b/src/mesa/drivers/dri/i965/brw_context.c
@@ -688,7 +688,7 @@ brw_initialize_context_constants(struct brw_context *brw)
/* ARB_viewport_array, OES_viewport_array */
if (devinfo->gen >= 6) {
ctx->Const.MaxViewports = GEN6_NUM_VIEWPORTS;
- ctx->Const.ViewportSubpixelBits = 0;
+ ctx->Const.ViewportSubpixelBits = 8;
/* Cast to float before negating because MaxViewportWidth is unsigned.
*/
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