OpenGL Shader stretches geomatry - c++

Question

I created a simple program that renders a quad
Initialize:
Math::float3 vertices[4];
vertices[0] = Math::float3(-0.5f, 0.5f, -1.0f);
vertices[1] = Math::float3(-0.5f, -0.5f, -1.0f);
vertices[2] = Math::float3(0.5f, -0.5f, -1.0f);
vertices[3] = Math::float3(0.5f, 0.5f, -1.0f);
glGenBuffers(1, &VBO);
glBindBuffer(GL_ARRAY_BUFFER, VBO);
glBufferData(GL_ARRAY_BUFFER, sizeof(vertices), vertices, GL_STATIC_DRAW);
unsigned int indices[] = {
0, 1, 2,
2, 3, 0
};
glGenBuffers(1, &IBO);
glBindBuffer(GL_ELEMENT_ARRAY_BUFFER, IBO);
glBufferData(GL_ELEMENT_ARRAY_BUFFER, sizeof(indices), indices, GL_STATIC_DRAW);
Rendering:
glEnableVertexAttribArray(0);
glBindBuffer(GL_ARRAY_BUFFER, VBO);
glVertexAttribPointer(0, 3, GL_FLOAT, false, sizeof(Math::float3), 0);
glBindBuffer(GL_ELEMENT_ARRAY_BUFFER, IBO);
glDrawElements(GL_TRIANGLES, 6, GL_UNSIGNED_INT, 0);
glDisableVertexAttribArray(0);
Which renders a perfectly square quad.
I continued by adding a vertex shader program:
Vertex shader
layout (location = 0) in vec3 Position;
void main(){
gl_Position = vec4(Position.x, Position.y, Position.z, 1.0);
}
Fragment shader
out vec4 FragColor;
void main()
{
FragColor = vec4(1.0, 0.0, 0.0, 1.0);
}
And if I run my program again, the quad seems to be stretched to about twice its size on x axis, and stretched slightly on the Y.
Without shaders
With shaders
Any idea on what could be causing this? If you need more code snippets or additional info, please let me know :)
Update
Some extra code.
Opening the window / reshape
glutInit(&argc, argv);
glutInitDisplayMode(GLUT_DEPTH | GLUT_DOUBLE | GLUT_RGBA);
glutInitWindowPosition(0, 0);
glutInitWindowSize(SCREEN_WIDTH, SCREEN_HEIGHT);
glutCreateWindow(TITLE);
...
if(_h == 0) _h = 1;
float ratio = 1.0f * _w / _h;
glMatrixMode(GL_PROJECTION);
glLoadIdentity();
glViewport(0, 0, _w, _h);
gluPerspective(45.0f, ratio, 1, 1000);
glMatrixMode(GL_MODELVIEW);
Update 2
My problem sadly still persists, some extra info here:
My geomatry:
Math::float3 vertices[4];
vertices[0] = Math::float3(-0.5, -0.5f, -1.0f);
vertices[1] = Math::float3(0.5f, -0.5f, -1.0f);
vertices[2] = Math::float3(0.5f, 0.5f, -1.0f);
vertices[3] = Math::float3(-0.5f, 0.5f, -1.0f);
unsigned int indices[] = { 0, 1, 2, 2, 3, 0 };
My vertex shader
#version 330
layout (location = 0) in vec3 Position;
uniform mat4 MVP;
void main()
{
gl_Position = MVP * vec4(Position.x, Position.y, Position.z, 1.0);
}
Setup view
#define SCREEN_WIDTH 800
#define SCREEN_HEIGHT 600
#define FOV 60.0f
#define RATIO 1.0f * SCREEN_WIDTH / SCREEN_HEIGHT
#define ZNEAR 1.0f
#define ZFAR 1000.0f
if(_h == 0) _h = 1;
glMatrixMode(GL_PROJECTION);
glLoadIdentity();
glViewport(0, 0, _w, _h);
gluPerspective(FOV, RATIO, ZNEAR, ZFAR);
glMatrixMode(GL_MODELVIEW);
Initialize
Math::float3 vertices[4];
vertices[0] = Math::float3(-0.5, -0.5f, -1.0f);
vertices[1] = Math::float3(0.5f, -0.5f, -1.0f);
vertices[2] = Math::float3(0.5f, 0.5f, -1.0f);
vertices[3] = Math::float3(-0.5f, 0.5f, -1.0f);
glGenBuffers(1, &VBO);
glBindBuffer(GL_ARRAY_BUFFER, VBO);
glBufferData(GL_ARRAY_BUFFER, sizeof(vertices), vertices, GL_STATIC_DRAW);
unsigned int indices[] = { 0, 1, 2, 2, 3, 0 };
glGenBuffers(1, &IBO);
glBindBuffer(GL_ELEMENT_ARRAY_BUFFER, IBO);
glBufferData(GL_ELEMENT_ARRAY_BUFFER, sizeof(indices), indices, GL_STATIC_DRAW);
sl = new Loader::ShaderLoader();
sl->Initialize();
sl->AddShader("data/shaders/shader.vs", VERTEX_SHADER);
sl->AddShader("data/shaders/shader.fs", FRAGMENT_SHADER);
sl->CompileShaders();
Render
MVP = glGetUniformLocation(sl->GetShaderProgram(), "MVP");
float m[16];
f -= 0.01f;
m[0] = 1.0f;
m[1] = 0.0f;
m[2] = 0.0f;
m[3] = 0.0f;
m[4] = 0.0f;
m[5] = 1.0f;
m[6] = 0.0f;
m[7] = 0.0f;
m[8] = 0.0f;
m[9] = 0.0f;
m[10] = 1.0f;
m[11] = 0.0f;
m[12] = 0.0f;
m[13] = 0.0f;
m[14] = 0.0f;
m[15] = 1.0f;
glUniformMatrix4fv(MVP, 1, GL_TRUE, m);
glEnableVertexAttribArray(0);
glBindBuffer(GL_ARRAY_BUFFER, VBO);
glVertexAttribPointer(0, 3, GL_FLOAT, false, 0, 0);
glBindBuffer(GL_ELEMENT_ARRAY_BUFFER, IBO);
glDrawElements(GL_TRIANGLES, 6, GL_UNSIGNED_INT, 0);
glDisableVertexAttribArray(0);
I've also printed the value for MVP per render call, and it's always 0 for some reason.
Again, any additional tips are welcome, thanks!

Answer 1

Most likely you have a non-identity transformation chain (non-identity projection and/or modelview matrix) set up with the fixed function. When using shaders the fixed function matrices are not used, which may explain the difference.

Answer 2

This is caused by different approach to PVM (projection-view-model) matrix in OpenGL. In fixed pipeline, this matrix was defined by functions glMatrixMode.
When shaders are used, these matrices are usually sent to vertex shader as uniform and handled here.
Your vertex shader then will be something like this:
in vec3 Position;
uniform mat4 Projection;
uniform mat4 View;
uniform mat4 Model;
void main(){
gl_Position = Projection * View * Model * vec4(Position.x, Position.y, Position.z, 1.0);
}
This should provide some more info about these three matrices: Tutorial 3 matrices