MeshTools: added fullScreenTriangle() utility function.

I got bored of copy/pasting and redocumenting/redebugging this code in
every full screen effect ever implemented.

src/MeshTools/CMakeLists.txt

@@ -25,6 +25,7 @@
 # Files shared between main library and unit test library
 set(MagnumMeshTools_SRCS
     CompressIndices.cpp
+    FullScreenTriangle.cpp
     Tipsify.cpp)
 
 # Files compiled with different flags for main library and unit test library
@@ -37,6 +38,7 @@ set(MagnumMeshTools_HEADERS
     CompressIndices.h
     Duplicate.h
     FlipNormals.h
+    FullScreenTriangle.h
     GenerateFlatNormals.h
     Interleave.h
     RemoveDuplicates.h

src/MeshTools/FullScreenTriangle.cpp

@@ -0,0 +1,59 @@
+/*
+    This file is part of Magnum.
+
+    Copyright © 2010, 2011, 2012, 2013 Vladimír Vondruš <mosra@centrum.cz>
+
+    Permission is hereby granted, free of charge, to any person obtaining a
+    copy of this software and associated documentation files (the "Software"),
+    to deal in the Software without restriction, including without limitation
+    the rights to use, copy, modify, merge, publish, distribute, sublicense,
+    and/or sell copies of the Software, and to permit persons to whom the
+    Software is furnished to do so, subject to the following conditions:
+
+    The above copyright notice and this permission notice shall be included
+    in all copies or substantial portions of the Software.
+
+    THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+    IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+    FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL
+    THE AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+    LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING
+    FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER
+    DEALINGS IN THE SOFTWARE.
+*/
+
+#include "FullScreenTriangle.h"
+
+#include "Math/Vector2.h"
+#include "AbstractShaderProgram.h"
+#include "Buffer.h"
+#include "Context.h"
+#include "Mesh.h"
+
+namespace Magnum { namespace MeshTools {
+
+std::pair<Buffer, Mesh> fullScreenTriangle() {
+    Mesh mesh;
+    mesh.setPrimitive(Mesh::Primitive::Triangles)
+        .setVertexCount(3);
+
+    Buffer buffer;
+    #ifndef MAGNUM_TARGET_GLES
+    if(!Context::current()->isVersionSupported(Version::GL300))
+    #else
+    if(!Context::current()->isVersionSupported(Version::GLES300))
+    #endif
+    {
+        constexpr Vector2 triangle[] = {
+            Vector2(-1.0,  1.0),
+            Vector2(-1.0, -3.0),
+            Vector2( 3.0,  1.0)
+        };
+        buffer.setData(triangle, Buffer::Usage::StaticDraw);
+        mesh.addVertexBuffer(buffer, 0, AbstractShaderProgram::Attribute<0, Vector2>());
+    }
+
+    return {std::move(buffer), std::move(mesh)};
+}
+
+}}

src/MeshTools/FullScreenTriangle.h

@@ -0,0 +1,87 @@
+#ifndef Magnum_MeshTools_FullScreenTriangle_h
+#define Magnum_MeshTools_FullScreenTriangle_h
+/*
+    This file is part of Magnum.
+
+    Copyright © 2010, 2011, 2012, 2013 Vladimír Vondruš <mosra@centrum.cz>
+
+    Permission is hereby granted, free of charge, to any person obtaining a
+    copy of this software and associated documentation files (the "Software"),
+    to deal in the Software without restriction, including without limitation
+    the rights to use, copy, modify, merge, publish, distribute, sublicense,
+    and/or sell copies of the Software, and to permit persons to whom the
+    Software is furnished to do so, subject to the following conditions:
+
+    The above copyright notice and this permission notice shall be included
+    in all copies or substantial portions of the Software.
+
+    THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+    IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+    FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL
+    THE AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+    LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING
+    FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER
+    DEALINGS IN THE SOFTWARE.
+*/
+
+/** @file
+ * @brief Function @ref Magnum::MeshTools::fullScreenTriangle()
+ */
+
+#include <utility>
+
+#include "Magnum.h"
+#include "MeshTools/magnumMeshToolsVisibility.h"
+
+namespace Magnum { namespace MeshTools {
+
+/**
+@brief Create full screen triangle mesh
+
+Returns pre-configured mesh along with vertex buffer which can be used for
+full-screen post-processing effects. The mesh is single triangle covering whole
+screen area (@f$ (-1, -1) - (1, 1) @f$ on both dimensions) and provides only
+vertex positions, as other attributes (such as texture coordinates) can be
+computed from them. The vertex positions are, in order:
+@f[
+    \begin{pmatrix} -1 \\ 1 \end{pmatrix},
+    \begin{pmatrix} -1 \\ -3 \end{pmatrix},
+    \begin{pmatrix} 3 \\ 1 \end{pmatrix}
+@f]
+
+On OpenGL 2.1 and OpenGL ES 2.0 the vertex positions are passed explicitly as
+attribute `0`, contained in the buffer. On OpenGL 3.0+ and OpenGL ES 3.0+ the
+mesh is attribute-less and the vertex positions can be computed using
+`gl_VertexID` builtin shader variable, thus the returned vertex buffer is empty
+and basically useless.
+
+Computing positions in vertex shader in a portable way might be done like this.
+For OpenGL 2.1 and OpenGL ES 2.0 you then need to bind the location of `position`
+attribute to `0`.
+@code
+#if (!defined(GL_ES) && __VERSION__ >= 130) || (defined(GL_ES) && __VERSION__ >= 300)
+#define NEW_GLSL
+#endif
+
+#ifndef NEW_GLSL
+attribute lowp vec4 position;
+#endif
+
+void main() {
+    #ifdef NEW_GLSL
+    gl_Position = vec4((gl_VertexID == 2) ?  3.0 : -1.0,
+                       (gl_VertexID == 1) ? -3.0 :  1.0, 0.0, 1.0);
+    #else
+    gl_Position = position;
+    #endif
+}
+@endcode
+
+@attention The implementation needs to check OpenGL version, so it expects
+    active context.
+*/
+std::pair<Buffer, Mesh> MAGNUM_MESHTOOLS_EXPORT fullScreenTriangle();
+
+}}
+
+#endif