Draw points with integer coordinates in OpenGL 3.3?

Question

I know Normalized Device Coordinates a little and I know when I use float between -1.0 and 1.0, I can get the output.

However, when I want to use integers as vertex's position attribute, I can't get any rendering output. I have tried to use GL_INT and GL_TRUE in glVertexAttribPointer but it doesn't work.

eg.

std::vector<GLint> vex =
{
    0, 0, 0,
    4, 5, 0
};
glBufferData(GL_ARRAY_BUFFER, vex.size() * sizeof(GLint), vex.data(), GL_STATIC_DRAW);
glVertexAttribPointer(0, 3, GL_INT, GL_TRUE, 3 * sizeof(GLint), (void*)0);

// in the render loop
glBindVertexArray(VAO);
glDrawArrays(GL_LINES, 0, 2);

I use a basic vertex shader as follow:

#version 330 core
layout (location = 0) in vec3 aPos;
void main()
{
    gl_Position = vec4(aPos.x, aPos.y, aPos.z, 1.0);
}

I considered the GL_TRUE will normalize the integer positions into [-1.0, 1.0].

Maybe I ignored something important. So how can I render my point using integer coordinates correctly?

About glVertexAttribPointer() I have read this reference while I still can't get what I want.

Answer 1

However, when I want to use integers as vertex's position attribute, I can't get any rendering output.

You get not output, because the two points are too close together.

See OpenGL 4.6 API core profile specification; 2.3.5.1 Conversion from Normalized Fixed-Point to Floating-Point; page 25

Signed normalized fixed-point integers represent numbers in the range [−1, 1]. The conversion from a signed normalized fixed-point value c to the corresponding floating-point value f is performed using

f = max( c / (2^(b-1) - 1), -1.0 )

(c is the integral value, and b is the number of bits in the integral data format)

This means, for the data type GLint, 4 results in floating point 0.000000001862645 and 5 results in floating point 0.000000002328306.

Test your code, by using GLbyte instead of GLint. The following code results in a diagonal line across the viewport:

std::vector<GLbyte> vex = {  127,  127, 0,
                            -128, -128, 0 };

glBufferData(GL_ARRAY_BUFFER, vex.size() * sizeof(GLbyte), vex.data(), GL_STATIC_DRAW);
glVertexAttribPointer(0, 3, GL_BYTE, GL_TRUE, 0, (void*)0);