[Bug 70327] New: Casting floating point variable to integer not working properly while constant gets converted properly - bugzilla-daemon

From: bugzilla-daemon@freedesktop.org
To: dri-devel@lists.freedesktop.org
Subject: [Bug 70327] New: Casting floating point variable to integer not working properly while constant gets converted properly
Date: Wed, 09 Oct 2013 21:17:52 +0000	[thread overview]
Message-ID: <bug-70327-502@http.bugs.freedesktop.org/> (raw)

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https://bugs.freedesktop.org/show_bug.cgi?id=70327

          Priority: medium
            Bug ID: 70327
          Assignee: dri-devel@lists.freedesktop.org
           Summary: Casting floating point variable to integer not working
                    properly while constant gets converted properly
          Severity: normal
    Classification: Unclassified
                OS: Linux (All)
          Reporter: tony.wasserka@dolphin-emu.org
          Hardware: All
            Status: NEW
           Version: git
         Component: Drivers/Gallium/r600
           Product: Mesa

Created attachment 87353
  --> https://bugs.freedesktop.org/attachment.cgi?id=87353&action=edit
Full rendered scene with bug visible

I've recently been working on converting the shaders used by the Dolphin
emulator to use integers instead of floating point numbers (for better
emulation accuracy). This seems to have exposed a bug in the r600g driver
possibly related to float->int casting.

The core of the issue is this GLSL code line:
"iprev.rgb = (iprev.rgb * (int(256.0-fog))) / 256;" (*)

where fog has been initialized before as 
"float fog = clamp(ze - fog_uniform.z, 0.0, 1.0);"

The point is, while "fog" seems to be zero (a "fog==0.0" condition in the code
will return true), the pixel shader result of (*) is different than the one
that I get by substituting "fog" with 0.0. This gets very apparent in the
rendered scene, cf

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