OpenGL instanced array drawing - opengl

Question

I have a simple OpenGL program and trying to draw an instanced array that stored in a vertex shader. I'm using two follow shaders for rendering:
Vertex Shader:
#version 330 core
uniform mat4 MVP;
const int VertexCount = 4;
const vec2 Position[VertexCount] = vec2[](
vec2(-100.0f, -100.0f),
vec2( -100.0f, 100.0f),
vec2( 100.0f, -100.0f),
vec2(100.0f, 100.0f));
void main()
{
gl_Position = MVP * vec4(Position[gl_VertexID], 0.0, 1.0);
}
Fragment Shader:
#version 330 core
#define FRAG_COLOR 0
layout(location = FRAG_COLOR, index = 0) out vec4 Color;
void main()
{
Color = vec4(0, 1, 0, 1); //let it will be green.
}
After I've compiled and validated these shader I create a vertex array object and draw it like triangle strips:
glUseProgram(programHandle); //handle is checked and valid.
glBindVertexArray(vao);
glDrawArraysInstanced(GL_TRIANGLE_STRIP, 0, 4, 1);
The viewport of drawing is set to the window size like glViewport(0, 0, 800, 600). I pass to MVP a simple orthographic matrix with fallow code:
glUniformMatrix4fv(handle, 1, GL_FALSE, (GLfloat*)&matrix); //handle is checked and valid
where the matrix was initialized:
Matrix::CreateOrthographicOffCenter(-200, 200, -200, 200, 1.0f, -1.0f, &matrix);
...
void Matrix::CreateOrthographicOffCenter(float left, float right, float bottom, float top, float zNearPlane, float zFarPlane, Matrix* matrix)
{
memset(matrix, 0, sizeof(Matrix));
matrix->M11 = 2.0f / (right - left);
matrix->M14 = (-right - left) / (right - left);
matrix->M22 = 2.0f / (top - bottom);
matrix->M24 = (-top - bottom) / (top - bottom);
matrix->M33 = 1.0f / (zFarPlane - zNearPlane);
matrix->M34 = (-zNearPlane) / (zFarPlane - zNearPlane);
matrix->M44 = 1.0f;
}
The problem is I got no triangle strips on my screen. I tried to draw vertex without MVP matrix (gl_Position = vec4(Position[gl_VertexID], 0.0, 1.0)) but also got nothing. How to detect where the problem is?

Answer 1

glBindVertexArray(vao);
glDrawArraysInstanced(GL_TRIANGLE_STRIP, 0, 4, 1);
And what exactly is stored in that VAO? I'm guessing your answer will be "nothing."
If so, then you have run afoul of several problems. If this is a compatibility context (or GL 2.1 or before), then OpenGL does not allow you to render with a VAO that has nothing in it. That is, you can't render with all attributes disabled. You will get a GL_INVALID_OPERATION error.
However, if you are in a core context 3.2 or above, then you can render with a disabled VAO.
Of course, that's just what the OpenGL specification says. What NVIDIA's drivers say is that you can render with a disabled VAO in both core and compatibility. What ATI's drivers say is that you can't render with a disabled VAO in both core and compatibility.
In short, if you want your code to work, bind something. Enable an array and put a buffer object there. It doesn't matter what is in it, since your shader simply won't care. But if you want it to work on different implementations, bind something.