All surfaces follow D3D Y convention

src/compiler/OutputHLSL.cpp

 //
 // Copyright (c) 2002-2010 The ANGLE Project Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 //
 
 #include "compiler/OutputHLSL.h"
 
 #include "compiler/debug.h"
 #include "compiler/InfoSink.h"
@@ skipping 177 matching lines @@
 
         if (mUsesFrontFacing)
         {
             out << "uniform bool dx_PointsOrLines;\n"
                    "uniform bool dx_FrontCCW;\n";
         }
 ········
         out << "\n";
         out <<  uniforms;
         out << "\n";
 

[dgkoch, 2010/12/23 20:19:23]

It would be worth including a comment here explaining the reasoning for the
twiddling of the y-texcoord.  Might also be helpful to include the derivations
as well, particularly for some of the more exotic cases such as 2DProj and Cube
cases.

[apatrick1, 2011/01/07 23:40:22]

Done.

+        // The texture fetch functions "flip" the Y coordinate in one way or another. This is because textures are stored
+        // according to the OpenGL convention, i.e. (0, 0) is "bottom left", rather than the D3D convention where (0, 0)
+        // is "top left". Since the HLSL texture fetch functions expect textures to be stored according to the D3D
+        // convention, the Y coordinate passed to these functions is adjusted to compensate.
+        //
+        // The simplest case is texture2D where the mapping is Y -> 1-Y, which maps [0, 1] -> [1, 0].
+        //
+        // The texture2DProj functions are more complicated because the projection divides by either Z or W. For the vec3
+        // case, the mapping is Y -> Z-Y or Y/Z -> 1-Y/Z, which again maps [0, 1] -> [1, 0].
+        //
+        // For cube textures the mapping is Y -> -Y, which maps [-1, 1] -> [1, -1]. This is not sufficient on its own for the
+        // +Y and -Y faces, which are now on the "wrong sides" of the cube. This is compensated for by exchanging the
+        // +Y and -Y faces everywhere else throughout the code.
+········
         if (mUsesTexture2D)
         {
             out << "float4 gl_texture2D(sampler2D s, float2 t)\n"
                    "{\n"
                    "    return tex2D(s, float2(t.x, 1 - t.y));\n"
                    "}\n"
                    "\n";
         }
 
         if (mUsesTexture2D_bias)
@@ skipping 1976 matching lines @@
     if (string.substr(0, 3) != "gl_" && string.substr(0, 3) != "dx_")
     {
         return "_" + string;
     }
     else
     {
         return string;
     }
 }
 }