Primitives: hint that a solid strip cube can be made entirely in GLSL.

src/Magnum/Primitives/Cube.h

@@ -56,7 +56,11 @@ with @ref VertexFormat::Vector3 positions. The returned instance references
 data stored in constant memory. No normals or anything else --- use
 @ref cubeSolid() instead if you need these.
 
-@image html primitives-cubesolid.png width=256px
+Vertex positions of this mesh can be also generated directly in the vertex
+shader using @glsl gl_VertexID @ce ([source](https://twitter.com/turanszkij/status/1141638406956617730),
+adapted to exactly match the output of this function):
+
+@snippet Magnum/Primitives/Test/data.glsl cubeSolidStrip
 
 @see @ref cubeWireframe(), @ref MeshTools::generateTriangleStripIndices()
 */

src/Magnum/Primitives/Test/CubeTest.cpp

@@ -26,6 +26,7 @@
 #include <Corrade/TestSuite/Tester.h>
 
 #include "Magnum/Mesh.h"
+#include "Magnum/Math/Functions.h"
 #include "Magnum/Math/Vector3.h"
 #include "Magnum/Primitives/Cube.h"
 #include "Magnum/Trade/MeshData.h"
@@ -37,12 +38,14 @@ struct CubeTest: TestSuite::Tester {
 
     void solid();
     void solidStrip();
+    void solidStripGlsl();
     void wireframe();
 };
 
 CubeTest::CubeTest() {
     addTests({&CubeTest::solid,
               &CubeTest::solidStrip,
+              &CubeTest::solidStripGlsl,
               &CubeTest::wireframe});
 }
 
@@ -72,6 +75,25 @@ void CubeTest::solidStrip() {
         (Vector3{-1.0f, -1.0f, -1.0f}));
 }
 
+void CubeTest::solidStripGlsl() {
+    Trade::MeshData cube = Primitives::cubeSolidStrip();
+
+    /* Yes, really. */
+    auto solidStripVertex = [](UnsignedInt gl_VertexID) {
+        typedef Vector3 vec3;
+        #define mix Math::lerp
+        #include "data.glsl"
+        #undef mix
+        return corner;
+    };
+
+    auto vertices = cube.attribute<Vector3>(Trade::MeshAttribute::Position);
+    for(UnsignedInt i = 0; i != cube.vertexCount(); ++i) {
+        CORRADE_ITERATION(i);
+        CORRADE_COMPARE(solidStripVertex(i), vertices[i]);
+    }
+}
+
 void CubeTest::wireframe() {
     Trade::MeshData cube = Primitives::cubeWireframe();
 

src/Magnum/Primitives/Test/data.glsl

@@ -0,0 +1,31 @@
+/*
+    This file is part of Magnum.
+
+    Copyright © 2010, 2011, 2012, 2013, 2014, 2015, 2016, 2017, 2018, 2019
+              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.
+*/
+
+/* [cubeSolidStrip] */
+int b = 1 << gl_VertexID;
+vec3 corner = vec3(mix(-1.0, 1.0, bool(0xd785 & b)),
+                   mix(-1.0, 1.0, bool(0x31e3 & b)),
+                   mix(-1.0, 1.0, bool(0x02af & b)));
+/* [cubeSolidStrip] */