[Bug 35207] Rendering issues in Outlast

[wine-bugs at winehq.org]

http://bugs.winehq.org/show_bug.cgi?id=35207

--- Comment #12 from Stefan Dösinger <stefan at codeweavers.com> ---
I have compared the assembly generated from the bad and good shaders and I have
an idea what is going on. The game has some code similar to this:

float mul_plus_one(float a, float b)
{
    float c = a + 1.0;
    return c * b;
}

With the 1.0 not hardcoded in the shader code, the compiler generates code like
this

ADD c, a, {uniform_holding_one}
MUL ret, c, b

With the 1.0 known at compile time, the code looks like this:

MAD ret, a, b, b

which calculates ret = a * b + b. I guess that's cheaper in the hardware, and
mathematically it's equivalent.

the problem happens with

mul_plus_one(0.0, INF);

in the first case that's INF * 1.0 = INF. I the second case that's INF * 0.0 +
INF = NaN + INF = NaN. The NaN results in a 0.0 alpha output from the shader.
whereas INF would be 1.0. A later shader reads the texture, calculates 1 -
alpha and writes the result to the screen.

I haven't confirmed that this is what's going on beyond a doubt, but I'm quite
convinced that this is the problem here. So the problem is essentially bug
26967. (I think 26967 shouldn't have been closed INVALID just because it's
fixed on the nvidia drivers by luck.)

-- 
Do not reply to this email, post in Bugzilla using the
above URL to reply.
You are receiving this mail because:
You are watching all bug changes.