Triangle doesn't render [duplicate] - c++

Question

This question already has answers here:
How can I make OpenGL draw something using VBOs in XCODE?
(3 answers)
Closed 8 years ago.
I am having trouble rendering a simple triangle. The code below compiles and runs, except there isn't any triangle; only a black background.
GLuint VBO;
static void RenderSceneCB()
{
//glClear sets the bitplane area of the window to values previously selected by glClearColor, glClearDepth, and glClearStencil.
glClear(GL_COLOR_BUFFER_BIT);
glEnableVertexAttribArray(0);
glBindBuffer(GL_ARRAY_BUFFER, VBO);
glVertexAttribPointer(0, 3, GL_FLOAT, GL_FALSE, 0, 0);
glDrawArrays(GL_TRIANGLES, 0, 3);
glDisableVertexAttribArray(0);
//swaps the buffers of the current window if double buffered.
glutSwapBuffers();
}
static void InitializeGlutCallbacks()
{
glutDisplayFunc(RenderSceneCB);
}
static void CreateVertexBuffer()
{
Vector3f Vertices[3];
Vertices[0] = Vector3f(-1.0f, -1.0f, 0.0f);
Vertices[1] = Vector3f(1.0f, -1.0f, 0.0f);
Vertices[2] = Vector3f(0.0f, 1.0f, 0.0f);
glGenBuffers(1, &VBO);
glBindBuffer(GL_ARRAY_BUFFER, VBO);
glBufferData(GL_ARRAY_BUFFER, sizeof(Vertices), Vertices, GL_STATIC_DRAW);
}
int main(int argc, char** argv)
{
glutInit(&argc, argv);
glutInitDisplayMode(GLUT_DOUBLE|GLUT_RGBA);
glutInitWindowSize(1024, 768);
glutInitWindowPosition(100, 100);
glutCreateWindow("Tutorial 02");
InitializeGlutCallbacks();
// Must be done after glut is initialized!
GLenum res = glewInit();
if (res != GLEW_OK) {
fprintf(stderr, "Error: '%s'\n", glewGetErrorString(res));
return 1;
}
glClearColor(0.0f, 0.0f, 0.0f, 0.0f);
CreateVertexBuffer();
glutMainLoop();
return 0;
}

Answer 1

Since you do not have a shader, OpenGL does not know how to interpret your vertex attribute 0, as it only knows about position, color, etc. and not about generic attributes. Note that this may work on some GPUs as they will map the generic attributes to the same "slots", and zero would then be interpreted as position (typically NVidia is less strict with these issues).
You can use MiniShader, just put the following code after CreateVertexBuffer();:
minish::InitializeGLExts(); // initialize shading extensions
const char *vs =
"layout(location = 0) in vec3 pos;\n"
// the layout specifier binds this variable to vertex attribute 0
"void main()\n"
"{\n"
" gl_Position = gl_ModelViewProjectionMatrix * vec4(pos, 1.0);\n"
"}\n";
const char *fs = "void main() { gl_FragColor=vec4(.0, 1.0, .0, 1.0); }"; // green
minish::CompileShader(vs, fs);
Note that I wrote that from top of my head, in case there are some errors, please comment.