title: OpenGL : second VBO spoiling first VBO
body: <p>I have a code that initially had one shader (ourShader), one VAO and one VBO. It would render a scene with a textured cube and its reflection on the ground using Stencil testing. Now, I wanted to add a framebuffer for post-processing so I needed a second shader. I added a second shader (screenShader) and a second VBO. I don't use them in my drawing yet, but the simple fact that I'm adding them makes my code render a black, red triangle instead of the usual scene.</p>
  
  <p>My code looks like this :</p>
  
  <pre><code>    Shader ourShader(string("core.vs"), string("core.frag")); // shader class creating a shader program from vertex shader and fragment shader source files.
      Shader screenShader(string("core2.vs"), string("core2.frag"));
  
      // Setting up attributes, VAO, VBO
      GLuint VAO;
      array&lt;GLuint, 2&gt; VBO;
      glGenVertexArrays(1, &amp;VAO);
      glGenBuffers(2, &amp;VBO[0]);
  
      glBindVertexArray(VAO);
  
      // Data for first shader and first VBO
      glBindBuffer(GL_ARRAY_BUFFER, VBO[0]);
      glBufferData(GL_ARRAY_BUFFER, sizeof(verticesCube), verticesCube, GL_STATIC_DRAW); 
  
      GLint posAttribLoc = glGetAttribLocation(ourShader.Program, "position");
      glVertexAttribPointer(posAttribLoc, 3, GL_FLOAT, GL_FALSE, 8 * sizeof(GLfloat), (GLvoid*)0); // specify (to the active VAO) how to retrieve the values for the attribute "position" from the data stored ("vertices" here) on the active VBO (GPU)
      glEnableVertexAttribArray(posAttribLoc); // enable attribute for rendering
  
      GLint colAttribLoc = glGetAttribLocation(ourShader.Program, "color");
      glVertexAttribPointer(colAttribLoc, 3, GL_FLOAT, GL_FALSE, 8 * sizeof(GLfloat), (GLvoid*)(3 * sizeof(GLfloat)));
      glEnableVertexAttribArray(colAttribLoc);
  
      GLint texAttribLoc = glGetAttribLocation(ourShader.Program, "texCoord");
      glVertexAttribPointer(texAttribLoc, 2, GL_FLOAT, GL_FALSE, 8 * sizeof(GLfloat), (GLvoid*)(6 * sizeof(GLfloat)));
      glEnableVertexAttribArray(texAttribLoc);
  
      // ##### PART GIVING A WEIRD RESULT #####
      // Data for second shader and second VBO
      glBindBuffer(GL_ARRAY_BUFFER, VBO[1]);
      glBufferData(GL_ARRAY_BUFFER, sizeof(verticesRectangle), verticesRectangle, GL_STATIC_DRAW);
  
      GLint posAttribLoc2 = glGetAttribLocation(screenShader.Program, "position");
      glVertexAttribPointer(posAttribLoc2, 2, GL_FLOAT, GL_FALSE, 4 * sizeof(GLfloat), (GLvoid*)0); // specify (to the active VAO) how to retrieve the values for the attribute "position" from the data stored ("vertices" here) on the active VBO (GPU)
      glEnableVertexAttribArray(posAttribLoc2); // enable attribute for rendering
  
      GLint texAttribLoc2 = glGetAttribLocation(screenShader.Program, "texCoord");
      glVertexAttribPointer(texAttribLoc2, 2, GL_FLOAT, GL_FALSE, 4 * sizeof(GLfloat), (GLvoid*)(2 * sizeof(GLfloat)));
      glEnableVertexAttribArray(texAttribLoc2);
     // ##### END #####
  
      // Setting up texture that will be used for the first shader
      GLuint texture;
      int width, height;
      glGenTextures(1, &amp;texture);
      glActiveTexture(GL_TEXTURE0);
      glBindTexture(GL_TEXTURE_2D, texture); // makes "texture" the current texture and attaches it to texture unit 0
  
      // Set the wrapping 
      glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_WRAP_S, GL_REPEAT); 
      glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_WRAP_T, GL_REPEAT);
  
      // Set the filtering 
      glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_MIN_FILTER, GL_LINEAR); 
      glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_MAG_FILTER, GL_LINEAR);
  
      unsigned char* image = SOIL_load_image("res/images/image1.jpg", &amp;width, &amp;height, 0, SOIL_LOAD_RGBA);
      glTexImage2D(GL_TEXTURE_2D, 0, GL_RGBA, width, height, 0, GL_RGBA, GL_UNSIGNED_BYTE, image);
      glGenerateMipmap(GL_TEXTURE_2D);
      SOIL_free_image_data(image);
  
      // Unbind VBO, texture before main loop
      glBindBuffer(GL_ARRAY_BUFFER, 0);
      glBindTexture(GL_TEXTURE_2D, 0); 
      glfwSwapInterval(1);
      glfwSetKeyCallback(window, Input::keyCallback); // Input is a singleton class handling inputs. It works well.
      glfwSetCursorPosCallback(window, Input::mouseCallback);
      glfwSetScrollCallback(window, Input::scrollCallback);
  
      while (glfwWindowShouldClose(window) == GLFW_FALSE) {
          // MAIN LOOP
          // ...
      }
      glDeleteVertexArrays(1, &amp;VAO);
      glDeleteBuffers(2, &amp;VBO[0]);
  </code></pre>
  
  <p>The mainloop is not important I think. What I want to say is that if I remove the few lines dealing with the the second VBO, then the scene renders well. Otherwise, I get a weird-colored triangle.</p>
  
  <p>And my two shaders use those source files for their respective vertex shader and fragment shader. By the way, I get no compilation errors from my shaders.</p>
  
  <p>core.vs :</p>
  
  <pre><code>#version 330 core
  in vec3 position;
  in vec2 texCoord;
  in vec3 color;
  
  out vec2 TexCoord;
  out vec3 Color;
  
  uniform mat4 model;
  uniform mat4 view;
  uniform mat4 projection;
  
  void main()
  {
      gl_Position = projection*view*model*vec4(position, 1.0);
      TexCoord = vec2(texCoord.x, 1.0 - texCoord.y);
      Color = color;
  }
  </code></pre>
  
  <p>core.frag :</p>
  
  <pre><code>#version 330 core
  in vec2 TexCoord;
  in vec3 Color;
  
  out vec4 outColor;
  uniform sampler2D ourTexture0;
  
  void main()
  {
      outColor = vec4(Color, 1)*texture(ourTexture0, TexCoord);
  }
  </code></pre>
  
  <p>core2.vs :</p>
  
  <pre><code>#version 330 core
  in vec2 position;
  in vec2 texCoord;
  out vec2 TexCoord;
  void main()
  {
      gl_Position = vec4(position, 0.0, 1.0);
      TexCoord = texCoord;
  }
  </code></pre>
  
  <p>core2.frag :</p>
  
  <pre><code>#version 330 core
  in vec2 TexCoord;
  out vec4 outColor;
  uniform sampler2D texFramebuffer;
  void main()
  {
      outColor = texture(texFramebuffer, TexCoord);
  }
  </code></pre>
  
  <p>The vertices look like this (but only the way to read them is important) :</p>
  
  <pre><code>    GLfloat verticesRectangle[] = {
          // position     // texture coordinates
          -0.5f, -0.5f,   0.0f, 0.0f,     
          0.5f, -0.5f,    1.0f, 0.0f,     
          -0.5f, 0.5f,    0.0f, 1.0f,     
          0.5f, 0.5f,     1.0f, 1.0f      
      };
  
      GLfloat verticesCube[] = {
          // position          // color          // texture coordinates
          -0.5f, -0.5f, -0.5f, 1.0f, 1.0f, 1.0f, 0.0f, 0.0f,
          0.5f, -0.5f, -0.5f, 1.0f, 1.0f, 1.0f, 1.0f, 0.0f,
          0.5f,  0.5f, -0.5f, 1.0f, 1.0f, 1.0f, 1.0f, 1.0f,
          0.5f,  0.5f, -0.5f, 1.0f, 1.0f, 1.0f, 1.0f, 1.0f,
          -0.5f,  0.5f, -0.5f, 1.0f, 1.0f, 1.0f, 0.0f, 1.0f,
          -0.5f, -0.5f, -0.5f, 1.0f, 1.0f, 1.0f, 0.0f, 0.0f,
  
          -0.5f, -0.5f,  0.5f, 1.0f, 1.0f, 1.0f, 0.0f, 0.0f,
          0.5f, -0.5f,  0.5f, 1.0f, 1.0f, 1.0f, 1.0f, 0.0f,
          0.5f,  0.5f,  0.5f, 1.0f, 1.0f, 1.0f, 1.0f, 1.0f,
          0.5f,  0.5f,  0.5f, 1.0f, 1.0f, 1.0f, 1.0f, 1.0f,
          -0.5f,  0.5f,  0.5f, 1.0f, 1.0f, 1.0f, 0.0f, 1.0f,
          -0.5f, -0.5f, 0.5f, 1.0f, 1.0f, 1.0f, 0.0f, 0.0f,
  
          -0.5f, 0.5f, 0.5f, 1.0f, 1.0f, 1.0f, 1.0f, 0.0f,
          -0.5f, 0.5f, -0.5f, 1.0f, 1.0f, 1.0f, 1.0f, 1.0f,
          -0.5f, -0.5f, -0.5f, 1.0f, 1.0f, 1.0f, 0.0f, 1.0f,
          -0.5f, -0.5f, -0.5f, 1.0f, 1.0f, 1.0f, 0.0f, 1.0f,
          -0.5f, -0.5f, 0.5f, 1.0f, 1.0f, 1.0f, 0.0f, 0.0f,
          -0.5f, 0.5f, 0.5f, 1.0f, 1.0f, 1.0f, 1.0f, 0.0f,
  
          0.5f, 0.5f, 0.5f, 1.0f, 1.0f, 1.0f, 1.0f, 0.0f,
          0.5f, 0.5f, -0.5f, 1.0f, 1.0f, 1.0f, 1.0f, 1.0f,
          0.5f, -0.5f, -0.5f, 1.0f, 1.0f, 1.0f, 0.0f, 1.0f,
          0.5f, -0.5f, -0.5f, 1.0f, 1.0f, 1.0f, 0.0f, 1.0f,
          0.5f, -0.5f, 0.5f, 1.0f, 1.0f, 1.0f, 0.0f, 0.0f,
          0.5f, 0.5f, 0.5f, 1.0f, 1.0f, 1.0f, 1.0f, 0.0f,
  
          -0.5f, -0.5f, -0.5f, 1.0f, 1.0f, 1.0f, 0.0f, 1.0f,
          0.5f, -0.5f, -0.5f, 1.0f, 1.0f, 1.0f, 1.0f, 1.0f,
          0.5f, -0.5f, 0.5f, 1.0f, 1.0f, 1.0f, 1.0f, 0.0f,
          0.5f, -0.5f, 0.5f, 1.0f, 1.0f, 1.0f, 1.0f, 0.0f,
          -0.5f, -0.5f, 0.5f, 1.0f, 1.0f, 1.0f, 0.0f, 0.0f,
          -0.5f, -0.5f, -0.5f, 1.0f, 1.0f, 1.0f, 0.0f, 1.0f,
  
          -0.5f, 0.5f, -0.5f, 1.0f, 1.0f, 1.0f, 0.0f, 1.0f,
          0.5f, 0.5f, -0.5f, 1.0f, 1.0f, 1.0f, 1.0f, 1.0f,
          0.5f, 0.5f, 0.5f, 1.0f, 1.0f, 1.0f, 1.0f, 0.0f,
          0.5f, 0.5f, 0.5f, 1.0f, 1.0f, 1.0f, 1.0f, 0.0f,
          -0.5f, 0.5f, 0.5f, 1.0f, 1.0f, 1.0f, 0.0f, 0.0f,
          -0.5f, 0.5f, -0.5f, 1.0f, 1.0f, 1.0f, 0.0f, 1.0f,
  
          -1.0f, -0.5f, 1.0f, 0.0f, 0.0f, 0.0f, 0.0f, 0.0f, // reflection surface // not a part of the cube itself
          -1.0f, -0.5f, -1.0f, 0.0f, 0.0f, 0.0f, 1.0f, 1.0f,
          1.0f, -0.5f, 1.0f, 0.0f, 0.0f, 0.0f, 0.0f, 1.0f,
          1.0f, -0.5f, -1.0f, 0.0f, 0.0f, 0.0f, 1.0f, 0.0f,
          -1.0f, -0.5f, -1.0f, 0.0f, 0.0f, 0.0f, 1.0f, 1.0f,
          1.0f, -0.5f, 1.0f, 0.0f, 0.0f, 0.0f, 0.0f, 0.0f
      };
  </code></pre>
  
  <p>I've already looked at <a href="https://stackoverflow.com/questions/12426507/binding-a-second-vertex-buffer-seems-to-spoil-my-first-vertex-buffer-opengl-oes">Binding a second vertex buffer seems to spoil my first vertex buffer, OpenGL OES ios 5.1</a> but the person had this problem because he didn't use his VAO correctly.</p>