View Full Version : Performance loss with glVertexAttribPointer

08-23-2011, 06:31 PM
I'm trying to save memory by using bytes to store texture and color information for my vertex format, however I've hit a performance problem when trying to use my vertex format with VBOs and a shader.

Here is my rendering code,

glBindBuffer( GL_ARRAY_BUFFER, chunkGeom->vertexBO );
glBindBuffer( GL_ELEMENT_ARRAY_BUFFER, chunkGeom->indexBO );

glVertexAttribPointer( attrTC, 2, GL_BYTE, GL_FALSE, sizeof(GLvertex), BUFFER_OFFSET(12) );
glVertexAttribPointer( attrL, 1, GL_BYTE, GL_FALSE, sizeof(GLvertex), BUFFER_OFFSET(15) );

glVertexPointer( 3, GL_FLOAT, sizeof(GLvertex), BUFFER_OFFSET(0) );

glDrawRangeElements( GL_TRIANGLES, 0, chunkGeom->vertexCount, chunkGeom->edgeCount, GL_UNSIGNED_SHORT, 0);

glBindBuffer( GL_ARRAY_BUFFER, 0 );
This gets run for the 400x objects in my scene, which are not overly complex as this code was running at 60 frames a second previously ( with vsync ), and is now running at around 11 frames a second.

Have I made some sort of mistake here that is resulting in poor perfomance?

here are my simple shaders, although I doubt the problem lies here

attribute vec2 in_texcoords;
attribute float in_light;

out vec2 f_texcoords;
out float f_light;

void main(void)
f_texcoords = in_texcoords;
f_light = in_light;
gl_Position = gl_ModelViewProjectionMatrix * gl_Vertex;

uniform sampler2D worldDiffuse;

in vec2 f_texcoords;
in float f_light;

void main (void)
gl_FragColor = texture2D( worldDiffuse, f_texcoords.xy * 0.25 );

Alfonse Reinheart
08-23-2011, 07:39 PM
One of the rules of thumb that I've always heard is to always make attributes start on 4-byte boundaries. So while you could make your texture coordinates shorts, you can't really make the light a byte.

Although one wonders why it is you're passing the light index per-vertex.

08-23-2011, 08:27 PM
the light value is intended to be the pre-computed brightness. I simply haven't multiplied it with the output in the fragment shader.

as for the alignments; the vertex format is currently

float x,y,z byte u,v byte brightness.
that fits neatly inside 16 bytes so I was hoping there was a way to keep the format small.

08-24-2011, 12:52 AM
The easiest way would be to use a vec3 attrib for (u,v,brightness) and separate it into two variables in the vertex shader.

That would be 4-byte aligned again.

08-24-2011, 02:35 AM
The easiest way would be to use a vec3 attrib for (u,v,brightness) and separate it into two variables in the vertex shader.

That would be 4-byte aligned again.
oh of course! I changed the format to simply bind 4 bytes as a vec4 as there's a spare byte and I'm back at 60FPS.

Thank you for your help.