Shaders: ported MeshVisualizer to not require geometry shader.

Wireframe width and smoothness isn't ported yet, because I don't fully
understand the behavior of standard derivatives in fragment shader.

src/Shaders/MeshVisualizer.cpp

@@ -33,35 +33,81 @@
 namespace Magnum { namespace Shaders {
 
 MeshVisualizer::MeshVisualizer(const Flags flags): flags(flags), transformationProjectionMatrixUniform(0), viewportSizeUniform(1), colorUniform(2), wireframeColorUniform(3), wireframeWidthUniform(4), smoothnessUniform(5) {
-    if(flags & Flag::Wireframe) MAGNUM_ASSERT_EXTENSION_SUPPORTED(Extensions::GL::ARB::geometry_shader4);
-    MAGNUM_ASSERT_EXTENSION_SUPPORTED(Extensions::GL::ARB::explicit_attrib_location);
-    MAGNUM_ASSERT_EXTENSION_SUPPORTED(Extensions::GL::ARB::explicit_uniform_location);
+    #ifndef MAGNUM_TARGET_GLES
+    if(flags & Flag::Wireframe && !(flags & Flag::NoGeometryShader))
+        MAGNUM_ASSERT_EXTENSION_SUPPORTED(Extensions::GL::ARB::geometry_shader4);
+    #endif
 
     Utility::Resource rs("MagnumShaders");
 
-    Shader vert(Version::GL330, Shader::Type::Vertex);
+    #ifndef MAGNUM_TARGET_GLES
+    const Version v = Context::current()->supportedVersion({Version::GL320, Version::GL310, Version::GL210});
+    #else
+    const Version v = Context::current()->supportedVersion({Version::GLES300, Version::GLES200});
+    #endif
+
+    Shader vert(v, Shader::Type::Vertex);
     vert.addSource(flags & Flag::Wireframe ? "#define WIREFRAME_RENDERING\n" : "")
+        .addSource(flags & Flag::NoGeometryShader ? "#define NO_GEOMETRY_SHADER\n" : "")
         .addSource(rs.get("compatibility.glsl"))
         .addSource(rs.get("MeshVisualizer.vert"));
     CORRADE_INTERNAL_ASSERT_OUTPUT(vert.compile());
+    vert.compile();
     attachShader(vert);
 
-    if(flags & Flag::Wireframe) {
-        Shader geom(Version::GL330, Shader::Type::Geometry);
+    #ifndef MAGNUM_TARGET_GLES
+    if(flags & Flag::Wireframe && !(flags & Flag::NoGeometryShader)) {
+        Shader geom(v, Shader::Type::Geometry);
         geom.addSource(rs.get("compatibility.glsl"))
             .addSource(rs.get("MeshVisualizer.geom"));
         CORRADE_INTERNAL_ASSERT_OUTPUT(geom.compile());
+        geom.compile();
         attachShader(geom);
     }
+    #endif
 
-    Shader frag(Version::GL330, Shader::Type::Fragment);
+    Shader frag(v, Shader::Type::Fragment);
     frag.addSource(flags & Flag::Wireframe ? "#define WIREFRAME_RENDERING\n" : "")
+        .addSource(flags & Flag::NoGeometryShader ? "#define NO_GEOMETRY_SHADER\n" : "")
         .addSource(rs.get("compatibility.glsl"))
         .addSource(rs.get("MeshVisualizer.frag"));
     CORRADE_INTERNAL_ASSERT_OUTPUT(frag.compile());
+    frag.compile();
     attachShader(frag);
 
+    #ifndef MAGNUM_TARGET_GLES
+    if(!Context::current()->isExtensionSupported<Extensions::GL::ARB::explicit_attrib_location>() ||
+        Context::current()->version() == Version::GL210)
+    #else
+    if(!Context::current()->isVersionSupported(Version::GLES300))
+    #endif
+    {
+        bindAttributeLocation(Position::Location, "position");
+        #ifndef MAGNUM_TARGET_GLES
+        if(v < Version::GL310)
+        #else
+        if(v < Version::GLES300)
+        #endif
+            bindAttributeLocation(VertexIndex::Location, "vertexIndex");
+    }
+
+    CORRADE_INTERNAL_ASSERT_OUTPUT(link());
     link();
+
+    #ifndef MAGNUM_TARGET_GLES
+    if(!Context::current()->isExtensionSupported<Extensions::GL::ARB::explicit_uniform_location>())
+    #endif
+    {
+        transformationProjectionMatrixUniform = uniformLocation("transformationProjectionMatrix");
+        colorUniform = uniformLocation("color");
+        if(flags & Flag::Wireframe) {
+            wireframeColorUniform = uniformLocation("wireframeColor");
+            wireframeWidthUniform = uniformLocation("wireframeWidth");
+            smoothnessUniform = uniformLocation("smoothness");
+            if(!(flags & Flag::NoGeometryShader))
+                viewportSizeUniform = uniformLocation("viewportSize");
+        }
+    }
 }
 
 }}

src/Shaders/MeshVisualizer.frag

@@ -22,24 +22,60 @@
     DEALINGS IN THE SOFTWARE.
 */
 
+#ifndef NEW_GLSL
+#define in varying
+#define fragmentColor gl_FragColor
+#endif
+
+#ifndef EXPLICIT_UNIFORM_LOCATION
+#define layout(arg)
+#endif
+
+#ifndef GL_ES
 layout(location = 2) uniform vec4 color = vec4(1.0, 1.0, 1.0, 1.0);
+#else
+uniform lowp vec4 color;
+#endif
 
 #ifdef WIREFRAME_RENDERING
+#ifndef GL_ES
 layout(location = 3) uniform vec4 wireframeColor = vec4(0.0, 0.0, 0.0, 1.0);
 layout(location = 4) uniform float wireframeWidth = 1.0;
 layout(location = 5) uniform float smoothness = 2.0;
+#else
+uniform lowp vec4 wireframeColor;
+uniform lowp float wireframeWidth;
+uniform lowp float smoothness;
+#endif
+
+#ifndef NO_GEOMETRY_SHADER
 noperspective in vec3 dist;
+#else
+in vec3 barycentric;
+#endif
 #endif
 
 out vec4 fragmentColor;
 
+#if defined(WIREFRAME_RENDERING) && defined(NO_GEOMETRY_SHADER) && defined(GL_ES)
+#extension GL_OES_standard_derivatives : enable
+#endif
+
 void main() {
     #ifdef WIREFRAME_RENDERING
+    #ifndef NO_GEOMETRY_SHADER
     /* Distance to nearest side */
     const float nearest = min(min(dist.x, dist.y), dist.z);
 
     /* Smooth step between face color and wireframe color based on distance */
     fragmentColor = mix(wireframeColor, color, smoothstep(wireframeWidth-smoothness, wireframeWidth+smoothness, nearest));
+    #else
+    const vec3 d = fwidth(barycentric);
+    const vec3 factor = smoothstep(vec3(0.0), d*1.5, barycentric);
+    const float nearest = min(min(factor.x, factor.y), factor.z);
+    fragmentColor = mix(wireframeColor, color, nearest);
+    #endif
+
     #else
     fragmentColor = color;
     #endif

src/Shaders/MeshVisualizer.h

@@ -39,23 +39,68 @@ namespace Magnum { namespace Shaders {
 /**
 @brief %Mesh visualization shader
 
-Uses geometry shader to visualize wireframe. You need to call at least
-setTransformationProjectionMatrix() (and setViewportSize(), if
-@ref Flag "Flag::Wireframe" is enabled) to be able to render.
-@requires_gl32 %Extension @extension{ARB,geometry_shader4} if
-    @ref Magnum::Shaders::MeshVisualizer::Flag "Flag::Wireframe" is enabled.
+Uses geometry shader to visualize wireframe. You need to provide @ref Position
+attribute in your triangle mesh and call at least @ref setTransformationProjectionMatrix()
+to be able to render.
+
+@section ShadersMeshVisualizer-wireframe Wireframe visualization
+
+Wireframe visualization is done by enabling @ref Flag "Flag::Wireframe". It is
+done either using geometry shaders or with help of additional vertex information.
+
+If you have geometry shaders available, you don't need to do anything else.
+
+@requires_gl32 %Extension @extension{ARB,geometry_shader4} for wireframe
+    rendering using geometry shaders.
+
+If you don't have geometry shaders, you need to set @ref Flag "Flag::NoGeometryShader"
+(it's enabled by default in OpenGL ES) and use only **non-indexed** triangle
+meshes (see MeshTools::duplicate() for possible solution). Additionaly, if you
+have OpenGL < 3.1 or OpenGL ES 2.0, you need to provide also @ref VertexIndex
+attribute.
+
+@requires_es_extension %Extension @extension{OES,standard_derivatives} for
+    wireframe rendering.
+
+@todo Understand and add support wireframe width/smoothness without GS
 */
 class MAGNUM_SHADERS_EXPORT MeshVisualizer: public AbstractShaderProgram {
     public:
         typedef Attribute<0, Vector3> Position; /**< @brief Vertex position */
 
+        /**
+         * @brief Vertex index
+         *
+         * Used only in OpenGL < 3.1 and OpenGL ES 2.0 if @ref Flag "Flag::Wireframe"
+         * is enabled. This attribute specifies index of given vertex in
+         * triangle, i.e. `0` for first, `1` for second, `2` for third. In
+         * OpenGL 3.1, OpenGL ES 3.0 and newer this value is provided by the
+         * shader itself, so the attribute is not needed.
+         */
+        typedef Attribute<1, Float> VertexIndex;
+
         /**
          * @brief %Flag
          *
          * @see Flags, MeshVisualizer()
          */
         enum class Flag: UnsignedByte {
-            Wireframe = 1 << 0      /**< Visualize wireframe */
+            /**
+             * Visualize wireframe. On OpenGL ES this also enables
+             * @ref Flag "Flag::NoGeometryShader".
+             */
+            #ifndef MAGNUM_TARGET_GLES
+            Wireframe = 1 << 0,
+            #else
+            Wireframe = (1 << 0) | (1 << 1),
+            #endif
+
+            /**
+             * Don't use geometry shader for wireframe visualization. If
+             * enabled, you might need to provide also VertexIndex attribute in
+             * the mesh. In OpenGL ES enabled alongside @ref Flag "Flag::Wireframe".
+             */
+            NoGeometryShader = 1 << 1
         };
 
         /** @brief %Flags */

src/Shaders/MeshVisualizer.vert

@@ -22,10 +22,41 @@
     DEALINGS IN THE SOFTWARE.
 */
 
+#ifndef NEW_GLSL
+#define in attribute
+#define out varying
+#endif
+
+#ifdef EXPLICIT_UNIFORM_LOCATION
 layout(location = 0) uniform mat4 transformationProjectionMatrix;
+#else
+uniform highp mat4 transformationProjectionMatrix;
+#endif
+
+#ifdef EXPLICIT_ATTRIB_LOCATION
+layout(location = 0) highp in vec4 position;
+#else
+in highp vec4 position;
+#endif
 
-layout(location = 0) in vec4 position;
+#if defined(WIREFRAME_RENDERING) && defined(NO_GEOMETRY_SHADER)
+#if (!defined(GL_ES) && __VERSION__ < 140) || (defined(GL_ES) && __VERSION__ < 300)
+#ifdef EXPLICIT_ATTRIB_LOCATION
+layout(location = 1) highp in float vertexIndex;
+#else
+in lowp float vertexIndex;
+#endif
+#define gl_VertexID int(vertexIndex)
+#endif
+
+out vec3 barycentric;
+#endif
 
 void main() {
     gl_Position = transformationProjectionMatrix*position;
+
+    #if defined(WIREFRAME_RENDERING) && defined(NO_GEOMETRY_SHADER)
+    barycentric = vec3(0.0);
+    barycentric[gl_VertexID % 3] = 1.0;
+    #endif
 }