[Mesa3d-dev] Re: Dramatic improvement of depth buffer quality

[Vladimir D. <vo...@mi...>]

Hi Felix - nice method :)

It would be nice to have this configurable on a per-application basis.

I imagine applications with a lot of detail in near scenes (autocad like
perhaps ?) would work better with the old way of doing things.

On the other hand, flight simulators are probably a prime candidates for=20
your way of doing things.

                      best

                         Vladimir Dergachev

On Fri, 31 Dec 2004, Felix [ISO-8859-1] K=FChling wrote:

> Hi,
>
> I want to share an idea that I had, which drastically improves the depth
> buffer quality on Savage4 hardware. Maybe the same can be applied to
> different hardware too.
>
> Short version: reverse the depth range (z' =3D 1 - z) such that far
> coordinates map to z'=3D0 and near coordinates to z'=3D1. Then use a
> floating point format to store depth values in the depth buffer. Of
> course the hardware needs to support a floating point depth buffer.
>
> By reversing the depth range a floating point format compensates for the
> usual loss of accuracy of depth values of far objects due to the
> perspective transformation. Reversing the depth range is done in the
> Driver.Viewport callback that computes the viewport transformation
> matrix. The depth compare functions have to be swapped (Less <->
> Greater, LessEqual <-> GreaterEqual) and the computation of the clear
> depth must be adjusted. See [0] for other adjustments that I havn't
> implemented yet.
>
> I have implemented this in the Savage driver and the result is really
> impressive. I had to reverse engineer the format of 16-bit float depth
> values used by the Savage4: it uses a 12 bit mantissa and a 4 bit
> unsigned exponent. A z-exponent offset can be programmed in order adjust
> it to the depth range of vertices (16 seems to be the right value for
> depth values in range [0,1]).
>
>   Format: eeee[0.]mmmmmmmmmmmm , Value: x =3D m * 2^(e-15)
>
> As a result, near objects are represented with only 12 bits (mantissa)
> instead of 16 bits, but this seems to be no problem. However, far
> objects are now represented 2^15 times more accurately than near objects
> and still 2^11 times more accurately than with a 16 bit fixed point
> format. (even up to 2^3 times more accurately that with a 24 bit fixed
> point depth buffer!)
>
> An interesting observation: reversing the depth range without using a
> floating point depth buffer already lead to a slight improvement of
> depth buffer quality. I guess this is due to how depth interpolation
> works internally on the Savage4.
>
> [0] There are still three problems I need to sort out: the float format
> used with a 24 bit depth buffer, polygon offsets (have to depend on the
> depth value) and correct handling of float depth values in software
> fallbacks.
>
> A happy new year to everybody!
>
> Bye,
>  Felix
>
> --=20
> | Felix K=FChling <fx...@gm...>                     http://fxk.de.vu |
> | PGP Fingerprint: 6A3C 9566 5B30 DDED 73C3  B152 151C 5CC1 D888 E595 |
>
>
>
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