Draw a 2D Quad in OpenGL - c++

Question

Whenever I think I understand something about openGL I find I don't understand anything.
I don't understand what I'm doing wrong.
The quad is not rendered.
Here is how I init the data
std::vector<glm::vec3> vertices;
vertices.resize(6);
// first triangle
vertices[0] = glm::vec3(x+w, y, 0.0f); // top right
vertices[1] = glm::vec3(x+w, y+h, 0.0f); // bottom right
vertices[2] = glm::vec3(x, y, 0.0f); // top left
// second triangle
vertices[3] = glm::vec3(x+w, y+h, 0.0f); // bottom right
vertices[4] = glm::vec3(x, y+h, 0.0f); // bottom left
vertices[5] = glm::vec3(x, y, 0.0f); // top left
Where (x, y) is coord of the top left corner in pixel screen position and (w, h) is the length of the sides of the quad
This is how I set in the GPU the data
glGenVertexArrays(1, &vao);
glBindVertexArray(vao);
glGenBuffers(1, &vbo);
glBindBuffer(GL_ARRAY_BUFFER, vbo);
glVertexAttribPointer(0, 3, GL_FLOAT, GL_FALSE, 0, nullptr);
glEnableVertexAttribArray(0);
glBufferData(GL_ARRAY_BUFFER, vertices.size() * sizeof(glm::vec3), vertices.data(), GL_STATIC_DRAW);
glBindVertexArray(0);
This is how I render it
glBindVertexArray(vao);
glPolygonMode(GL_FRONT_AND_BACK, GL_FILL);
glDrawArrays(GL_TRIANGLES, 0, 6);
glBindVertexArray(0);
And these are my vertex and fragment shader
#version 330 core
layout (location = 0) in vec3 position;
void main() {
gl_Position = vec4(position, 1.0);
}
#version 330 core
out vec4 outColor;
void main() {
outColor = vec4(1.0);
}
UPDATE
How Rabbit76 point out: I do not see the rectangle, because it is outside the view. If you don't use a projection matrix, the vertex coordinates must be in range [-1.0, 1.0]. If you want to use "window" coordinates you must use an orthographic projection"
So I changed the vertex shader in the following way and everything works!
#version 330 core
layout (location = 0) in vec3 position;
uniform vec2 viewport; //Width and Height of the viewport
void main() {
// From pixels to 0-1
vec2 coord = position.xy / viewport;
// Flip Y so that 0 is top
coord.y = (1.0-coord.y);
// Map to NDC -1,+1
coord.xy = coord.xy * 2.0 - 1.0;
gl_Position = vec4(coord.xy, 0.0, 1.0);
}

Answer 1

You don't see the rectangle because it's out of view. If you are not using a projection matrix, the vertex coordinates must be in the range [-1.0, 1.0]. If you want to use "window" coordinates you must use an Orthographic projection.
You need to transform the vertex coordinates through the projection matrix. This is usually done in the vertex shader. However, it is also possible to transform the vertex coordinates on the CPU. Use glm::ortho to define the orthographic projection matrix and transform the vertices:
glm::mat4 projection = glm::ortho(0.0f, window_width, window_height, 0.0f, -1.0f, 1.0f);
for (auto &vertex : vertices)
vertex = glm::vec3(projection * glm::vec4(vertex, 1.0f));