add RectShader; draw colorful background rectangle

rect.f.glsl

@@ -0,0 +1,12 @@
+#version 130
+
+/* Interpolated color */
+in vec4 i_color;
+
+/* Fragment color */
+out vec4 frag_color;
+
+void main(void)
+{
+    frag_color = i_color;
+}

rect.v.glsl

@@ -0,0 +1,31 @@
+#version 130
+
+/* Viewport width and height */
+uniform ivec2 viewport_size;
+/* Position offset */
+uniform ivec2 position;
+/* Rectangle size */
+uniform ivec2 size;
+
+/* Vertex coordinates: x, y */
+in vec2 coords;
+/* Vertex color */
+in vec4 color;
+
+/* Interpolated color */
+out vec4 i_color;
+
+/**
+ * Map coordinates such that:
+ * (0 .. viewport_size.[xy]) => (-1.0 .. 1.0)
+ */
+vec2 map_to_screen(vec2 position)
+{
+    return 2.0 * position / viewport_size - 1.0;
+}
+
+void main(void)
+{
+    gl_Position = vec4(map_to_screen(vec2(position) + coords * size), 0, 1);
+    i_color = color;
+}

src/RectShader.cc

@@ -0,0 +1,13 @@
+#include "RectShader.h"
+
+RectShader::RectShader()
+{
+    m_program = glcxx::Program::create(
+            glcxx::Shader::create_from_file(GL_VERTEX_SHADER, "rect.v.glsl"),
+            glcxx::Shader::create_from_file(GL_FRAGMENT_SHADER, "rect.f.glsl"),
+            "coords", 0);
+
+    m_uniforms.viewport_size = m_program->get_uniform_location("viewport_size");
+    m_uniforms.position = m_program->get_uniform_location("position");
+    m_uniforms.size = m_program->get_uniform_location("size");
+}

src/RectShader.h

@@ -0,0 +1,40 @@
+#ifndef RECTSHADER_H
+#define RECTSHADER_H
+
+#include "glcxx.hpp"
+#include <memory>
+
+class RectShader
+{
+public:
+    RectShader();
+
+    void use() { m_program->use(); }
+
+    void set_viewport_size(int width, int height)
+    {
+        glUniform2i(m_uniforms.viewport_size, width, height);
+    }
+
+    void set_position(int x, int y)
+    {
+        glUniform2i(m_uniforms.position, x, y);
+    }
+
+    void set_size(int x, int y)
+    {
+        glUniform2i(m_uniforms.size, x, y);
+    }
+
+protected:
+    std::shared_ptr<glcxx::Program> m_program;
+
+    struct
+    {
+        GLint viewport_size;
+        GLint position;
+        GLint size;
+    } m_uniforms;
+};
+
+#endif

src/main.cc

@@ -4,17 +4,21 @@
 #include "GL3/gl3w.h"
 #include <iostream>
 #include "TextShader.h"
+#include "RectShader.h"
 
 using namespace std;
 
 std::shared_ptr<TextShader> text_shader;
+std::shared_ptr<RectShader> rect_shader;
+std::shared_ptr<glcxx::Array> rect_array;
+std::shared_ptr<glcxx::Buffer> rect_buffer;
 
 #define WIDTH   800
 #define HEIGHT  800
 
 bool init(void)
 {
-    glClearColor (0.0, 0.0, 0.0, 0.0);
+    glClearColor(0.0, 0.0, 0.0, 0.0);
     glEnable(GL_CULL_FACE);
     glEnable(GL_BLEND);
     glBlendFunc(GL_SRC_ALPHA, GL_ONE_MINUS_SRC_ALPHA);
@@ -25,6 +29,26 @@ bool init(void)
         text_shader->use();
         text_shader->set_texture(0);
         text_shader->set_viewport_size(WIDTH, HEIGHT);
+
+        rect_shader = std::make_shared<RectShader>();
+        rect_shader->use();
+        rect_shader->set_viewport_size(WIDTH, HEIGHT);
+        rect_shader->set_position(0, 0);
+        rect_shader->set_size(WIDTH, HEIGHT);
+        GLfloat rect_buffer_data[] = {
+            0.0, 0.0,   0.0, 0.0, 1.0, 1.0,
+            1.0, 0.0,   1.0, 1.0, 1.0, 1.0,
+            1.0, 1.0,   0.0, 1.0, 0.0, 1.0,
+            0.0, 1.0,   1.0, 0.0, 0.0, 1.0,
+        };
+        rect_array = glcxx::Array::create();
+        rect_array->bind();
+        rect_buffer = glcxx::Buffer::create(GL_ARRAY_BUFFER, GL_STATIC_DRAW,
+                rect_buffer_data, sizeof(rect_buffer_data));
+        glEnableVertexAttribArray(0);
+        glEnableVertexAttribArray(1);
+        glVertexAttribPointer(0, 2, GL_FLOAT, GL_FALSE, 6 * sizeof(GLfloat), 0);
+        glVertexAttribPointer(1, 4, GL_FLOAT, GL_FALSE, 6 * sizeof(GLfloat), (void *)(2 * sizeof(GLfloat)));
     }
     catch (glcxx::Error & e)
     {
@@ -38,6 +62,10 @@ void display(SDL_Window * window)
 {
     glClear(GL_COLOR_BUFFER_BIT);
 
+    rect_array->bind();
+    rect_shader->use();
+    glDrawArrays(GL_TRIANGLE_FAN, 0, 4);
+
     SDL_GL_SwapWindow(window);
 }