OpenGL Vertex Shader not working

Question

I am learning trying to learn GLSL with Java and the LWJGL, but I am currently having problems with a basic Vertex Shader.

Vertex Shader:

#version 120

void main() {
    gl_Position = gl_Vertex * 0.5;
}

Fragment Shader:

#version 120

void main() {
    gl_FragColor = vec4(1.0, 0.4, 0.4, 1.0);
}

Rendering code:

public void draw() {
    glBindBuffer(GL_ARRAY_BUFFER, vbo);
    glVertexPointer(3, GL_FLOAT, 0, 0L);

    glBindBuffer(GL_ELEMENT_ARRAY_BUFFER, ibo);

    glEnableClientState(GL_VERTEX_ARRAY);
    glDrawElements(GL_TRIANGLES, size, GL_UNSIGNED_INT, 0);
    glDisableClientState(GL_VERTEX_ARRAY);
}

The compilation yields no errors and the Fragment Shader works fine, tinting a simple triangle I draw. The only problem is that I dont see the triangle downscaled (0.5 factor) as expected, it's vertices remain unaltered. What am I doing wrong?

gl_Position = gl_Vertex * 0.5 will modify the whole 4 dimensional vector including the w component. Do something like gl_Position = vec4(gl_Vertex.xyz * 0.5, 1.0) — Justin Meiners
– Justin Meiners, Commented Aug 22, 2013 at 20:04

Justin Meiners · Accepted Answer · 2013-08-22 20:46:19Z

2

The problem is that gl_Vertex is a 4 dimensional vector. Multiplying it directly with a scalar will not give you the scaling results intended. OpenGL divides the x,y,z with the w component when converting the homogeneous gl_Position to cartesian screen coordinates. 0.5 / 0.5 = 1.0 so the result is no scaling.

gl_Position = gl_Vertex * 0.5;

This multiples the x, y, z, and w components by 0.5. Change the code to only scale x, y, and z.

gl_Position = vec4(gl_Vertex.xyz * 0.5, gl_Vertex.w);

edited Aug 22, 2013 at 20:46

answered Aug 22, 2013 at 20:14

Justin Meiners

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