Tracking AbstractShaderProgram state.

Reduces OpenGL call count when subsequently using the same shader for
rendering.

src/AbstractShaderProgram.cpp

@@ -18,6 +18,8 @@
 #include <fstream>
 
 #include "Shader.h"
+#include "Implementation/State.h"
+#include "Implementation/ShaderProgramState.h"
 
 #define LINKER_MESSAGE_MAX_LENGTH 1024
 
@@ -25,10 +27,24 @@ using namespace std;
 
 namespace Magnum {
 
+AbstractShaderProgram::~AbstractShaderProgram() {
+    /* Remove current usage from the state */
+    GLuint& current = Context::current()->state()->shaderProgram->current;
+    if(current == _id)
+        current = 0;
+
+    glDeleteProgram(_id);
+}
+
 bool AbstractShaderProgram::use() {
     if(state != Linked) return false;
 
-    glUseProgram(_id);
+    /* Use only if the program isn't already in use */
+    GLuint& current = Context::current()->state()->shaderProgram->current;
+    if(current != _id) {
+        current = _id;
+        glUseProgram(_id);
+    }
     return true;
 }
 

src/AbstractShaderProgram.h

@@ -642,8 +642,6 @@ class MAGNUM_EXPORT AbstractShaderProgram {
         State state;
 };
 
-inline AbstractShaderProgram::~AbstractShaderProgram() { glDeleteProgram(_id); }
-
 }
 
 #endif

src/Implementation/ShaderProgramState.h

@@ -0,0 +1,31 @@
+#ifndef Magnum_Implementation_ShaderProgramState_h
+#define Magnum_Implementation_ShaderProgramState_h
+/*
+    Copyright © 2010, 2011, 2012 Vladimír Vondruš <mosra@centrum.cz>
+
+    This file is part of Magnum.
+
+    Magnum is free software: you can redistribute it and/or modify
+    it under the terms of the GNU Lesser General Public License version 3
+    only, as published by the Free Software Foundation.
+
+    Magnum is distributed in the hope that it will be useful,
+    but WITHOUT ANY WARRANTY; without even the implied warranty of
+    MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
+    GNU Lesser General Public License version 3 for more details.
+*/
+
+#include "Magnum.h"
+
+namespace Magnum { namespace Implementation {
+
+struct ShaderProgramState {
+    inline constexpr ShaderProgramState(): current(0) {}
+
+    /* Currently used program */
+    GLuint current;
+};
+
+}}
+
+#endif

src/Implementation/State.cpp

@@ -16,12 +16,14 @@
 #include "State.h"
 
 #include "BufferState.h"
+#include "ShaderProgramState.h"
 
 namespace Magnum { namespace Implementation {
 
-State::State(): buffer(new BufferState) {}
+State::State(): buffer(new BufferState), shaderProgram(new ShaderProgramState) {}
 
 State::~State() {
+    delete shaderProgram;
     delete buffer;
 }
 

src/Implementation/State.h

@@ -20,12 +20,14 @@
 namespace Magnum { namespace Implementation {
 
 struct BufferState;
+struct ShaderProgramState;
 
 struct State {
     State();
     ~State();
 
     BufferState* const buffer;
+    ShaderProgramState* const shaderProgram;
 };
 
 }}