From: Mikko Rasa <tdb@tdb.fi>
Date: Mon, 13 Aug 2012 17:47:53 +0000 (+0300)
Subject: Lookup functions to determine the orientation of cube map faces
X-Git-Url: http://git.tdb.fi/?p=libs%2Fgl.git;a=commitdiff_plain;h=2157e9320c60d561208dfdf6034e9a06781c9f30

Lookup functions to determine the orientation of cube map faces

The orientations are somewhat arbitary and not deducible with a simple
algorithm.  Writing the necessary conditional ladders everywhere would
be annoying.
---

diff --git a/source/texturecube.cpp b/source/texturecube.cpp
index 4a613a37..ddb0c1e0 100644
--- a/source/texturecube.cpp
+++ b/source/texturecube.cpp
@@ -5,25 +5,19 @@
 
 using namespace std;
 
-namespace {
+namespace Msp {
+namespace GL {
 
-// An array to facilitate looping through the cube faces
-Msp::GL::TextureCubeFace faces[6] =
+Vector3 TextureCube::directions[6] =
 {
-	Msp::GL::POSITIVE_X,
-	Msp::GL::NEGATIVE_X,
-	Msp::GL::POSITIVE_Y,
-	Msp::GL::NEGATIVE_Y,
-	Msp::GL::POSITIVE_Z,
-	Msp::GL::NEGATIVE_Z
+	Vector3(1, 0, 0),
+	Vector3(-1, 0, 0),
+	Vector3(0, 1, 0),
+	Vector3(0, -1, 0),
+	Vector3(0, 0, 1),
+	Vector3(0, 0, -1)
 };
 
-}
-
-
-namespace Msp {
-namespace GL {
-
 TextureCube::TextureCube():
 	Texture(GL_TEXTURE_CUBE_MAP),
 	size(0),
@@ -49,7 +43,7 @@ void TextureCube::allocate(unsigned level)
 		return;
 
 	for(unsigned i=0; i<6; ++i)
-		image(faces[i], level, get_base_pixelformat(ifmt), UNSIGNED_BYTE, 0);
+		image(enumerate_faces(i), level, get_base_pixelformat(ifmt), UNSIGNED_BYTE, 0);
 }
 
 void TextureCube::image(TextureCubeFace face, unsigned level, PixelFormat fmt, DataType type, const void *data)
@@ -78,5 +72,71 @@ unsigned TextureCube::get_level_size(unsigned level)
 	return size>>level;
 }
 
+TextureCubeFace TextureCube::enumerate_faces(unsigned i)
+{
+	switch(i)
+	{
+	case 0: return POSITIVE_X;
+	case 1: return NEGATIVE_X;
+	case 2: return POSITIVE_Y;
+	case 3: return NEGATIVE_Y;
+	case 4: return POSITIVE_Z;
+	case 5: return NEGATIVE_Z;
+	default: throw out_of_range("TextureCube::enumerate_faces");
+	}
+}
+
+const Vector3 &TextureCube::get_face_direction(TextureCubeFace face)
+{
+	switch(face)
+	{
+	case POSITIVE_X: return directions[0];
+	case NEGATIVE_X: return directions[1];
+	case POSITIVE_Y: return directions[2];
+	case NEGATIVE_Y: return directions[3];
+	case POSITIVE_Z: return directions[4];
+	case NEGATIVE_Z: return directions[5];
+	default: throw invalid_argument("TextureCube::get_face_direction");
+	}
+}
+
+const Vector3 &TextureCube::get_s_direction(TextureCubeFace face)
+{
+	switch(face)
+	{
+	case POSITIVE_X: return directions[5];
+	case NEGATIVE_X: return directions[4];
+	case POSITIVE_Y: return directions[0];
+	case NEGATIVE_Y: return directions[0];
+	case POSITIVE_Z: return directions[0];
+	case NEGATIVE_Z: return directions[1];
+	default: throw invalid_argument("TextureCube::get_s_direction");
+	}
+}
+
+const Vector3 &TextureCube::get_t_direction(TextureCubeFace face)
+{
+	switch(face)
+	{
+	case POSITIVE_X: return directions[3];
+	case NEGATIVE_X: return directions[3];
+	case POSITIVE_Y: return directions[4];
+	case NEGATIVE_Y: return directions[5];
+	case POSITIVE_Z: return directions[3];
+	case NEGATIVE_Z: return directions[3];
+	default: throw invalid_argument("TextureCube::get_t_direction");
+	}
+}
+
+Vector3 TextureCube::get_texel_direction(TextureCubeFace face, unsigned u, unsigned v)
+{
+	float s = (u+0.5f)*2.0f/size-1.0f;
+	float t = (v+0.5f)*2.0f/size-1.0f;
+	const Vector3 &fv = get_face_direction(face);
+	const Vector3 &sv = get_s_direction(face);
+	const Vector3 &tv = get_t_direction(face);
+	return Vector3(fv.x+s*sv.x, fv.y+t*tv.y, fv.z+s*sv.z+t*tv.z);
+}
+
 } // namespace GL
 } // namespace Msp
diff --git a/source/texturecube.h b/source/texturecube.h
index b15c1cff..0d23884f 100644
--- a/source/texturecube.h
+++ b/source/texturecube.h
@@ -4,6 +4,7 @@
 #include "datatype.h"
 #include "pixelformat.h"
 #include "texture.h"
+#include "vector.h"
 
 namespace Msp {
 namespace GL {
@@ -23,7 +24,8 @@ Cube map texture, consisting of six square faces.  All of the faces must be of
 the same size.  A cube map texture is addressed by three-dimensional texture
 coordinates, with a principal range of [-1, 1].  The face is first selected
 according to the largest coordinate, and the remaining two coordinates are used
-to sample the face image.
+to sample the face image.  The images are oriented so that the cross product of
+the s and t axes will point into the cube.
 
 All faces of a cube map texture must be allocated for it to be usable.
 
@@ -36,6 +38,8 @@ private:
 	unsigned size;
 	unsigned allocated;
 
+	static Vector3 directions[6];
+
 public:
 	TextureCube();
 
@@ -63,6 +67,23 @@ public:
 	unsigned get_size() const { return size; }
 private:
 	unsigned get_level_size(unsigned);
+
+public:
+	/** Translates indices into face constants.  Valid indices are between 0
+	and 5, inclusive. */
+	static TextureCubeFace enumerate_faces(unsigned);
+
+	/** Returns a vector pointing out of the face. */
+	static const Vector3 &get_face_direction(TextureCubeFace);
+
+	/** Returns a vector in the direction of the s axis of the face. */
+	static const Vector3 &get_s_direction(TextureCubeFace);
+
+	/** Returns a vector in the direction of the t axis of the face. */
+	static const Vector3 &get_t_direction(TextureCubeFace);
+
+	/** Returns a vector pointing to the center a texel. */
+	Vector3 get_texel_direction(TextureCubeFace, unsigned, unsigned);
 };
 
 } // namespace GL