lordmule
12-27-2010, 10:51 PM
Hello, I have recently tried reading back some computations of vector data on the GPU, but I can't seem to get the right numbers. So as a test, I started played around with just reading back vertex data and still could not get it right.
I figured the best way to readback a non-normalised vector, is to save it as a normalized vector with its length. In case the length is greater than 1, I pre-scale that value to the range [0,1]. So my test fragment shader is:
// transformed vertex
varying vec3 g_vertex;
// specify an scale to use eg 10
uniform float scale;
void main()
{
vec3 v;
// we let the 4th coord be the vector scale
float w = length(g_vertex) / scale;
v = normalize(g_vertex);
v = 0.5 * v + 0.5;
gl_FragColor = vec4(v, w);
}
reconstruction after readback:
float *pix = vertexmap + 4*i;
vertex[0] = (pix[0] - 0.5) * 2.0;
vertex[1] = (pix[1] - 0.5) * 2.0;
vertex[2] = (pix[2] - 0.5) * 2.0;
vertex[0] *= pix[3] * vmap_scale;
vertex[1] *= pix[3] * vmap_scale;
vertex[2] *= pix[3] * vmap_scale;
It's ok when the scale is close to one, but for every point further from (0,0,0) I start to get concentric circle effect based on the scale I choose. For example scale = 6, then there will be some bands where vertices will round to. I've tried in different spaces object/eye and get the same result, I might be going about it the wrong way. It's something about that 4th component not working right...
I've made 3 snaps of scale = 1, and 3 snaps of scale = 6
images (http://img24.imageshack.us/g/cmcapture1c.png/)
Any suggestions?
edit: re-rendered as a 3D point cloud
I figured the best way to readback a non-normalised vector, is to save it as a normalized vector with its length. In case the length is greater than 1, I pre-scale that value to the range [0,1]. So my test fragment shader is:
// transformed vertex
varying vec3 g_vertex;
// specify an scale to use eg 10
uniform float scale;
void main()
{
vec3 v;
// we let the 4th coord be the vector scale
float w = length(g_vertex) / scale;
v = normalize(g_vertex);
v = 0.5 * v + 0.5;
gl_FragColor = vec4(v, w);
}
reconstruction after readback:
float *pix = vertexmap + 4*i;
vertex[0] = (pix[0] - 0.5) * 2.0;
vertex[1] = (pix[1] - 0.5) * 2.0;
vertex[2] = (pix[2] - 0.5) * 2.0;
vertex[0] *= pix[3] * vmap_scale;
vertex[1] *= pix[3] * vmap_scale;
vertex[2] *= pix[3] * vmap_scale;
It's ok when the scale is close to one, but for every point further from (0,0,0) I start to get concentric circle effect based on the scale I choose. For example scale = 6, then there will be some bands where vertices will round to. I've tried in different spaces object/eye and get the same result, I might be going about it the wrong way. It's something about that 4th component not working right...
I've made 3 snaps of scale = 1, and 3 snaps of scale = 6
images (http://img24.imageshack.us/g/cmcapture1c.png/)
Any suggestions?
edit: re-rendered as a 3D point cloud