gl_PointCoord give always 0,0

[Cedric]

Hi,

I am trying to do some point sprite using a texture and sampling it
from
gl_PointCoord in the fragment shader.
But I always get a coord (0,0) so do I need to enable something to
have
uv between (0,0) (1,1) ?

vec4 c;
c = texture2D(TexUnit4, gl_PointCoord);


Cheers,
Cedric

--
Provide OpenGL, WebGL and OpenSceneGraph services
+33 659 598 614 Cedric Pinson mailto:cedric...@plopbyte.net
http://www.plopbyte.net

[Giles Thomas]

Hi Cedric,

On 3 September 2010 16:53, Cedric <tri...@gmail.com> wrote:

I am trying to do some point sprite using a texture and sampling it
from
gl_PointCoord in the fragment shader.
But I always get a coord (0,0) so do I need to enable something to
have
uv between (0,0) (1,1) ?

vec4 c;
c = texture2D(TexUnit4, gl_PointCoord);

Sorry if it's a silly question, but are you setting gl_PointSize?

Cheers,

Giles

--
Giles Thomas
gi...@giles.net
http://www.gilesthomas.com/
http://projectdirigible.com/
http://learningwebgl.com/

[Paul Brunt]

I had the same problem, in the end I hacked round the problem using gl_FragCoord to figure out what the gl_PointCoord should have been from the point size and position, more long winded and inefficient but worked just as well.

[trigrou]

Hi
Would you have an example I could have look online ?

Cheers,
Cedric

[Paul Brunt]

I don't have it available at the moment it's on another machine but of the top of my head it should be something like:

vertex shader:

attribute vec3 aPosition

attribute float aPointSize

uniform mat4 uViewMatrix

uniform mat4 uProjection

varying float vPointSize

varying vec2 vPosition

void main(){

vec4 position=uProjection * uViewMatrix * vec4(aPosition,1.0);

gl_PointSize=aPointSize;

gl_Position=position;

//send the point size though to the fragment shader

vPointSize=aPointSize;

//send the screen coord though to the fragment shader

vPosition=vec2(position.x/position.w,position.y/position.w);

}

fragment shader:

varying float vPointSize

varying vec2 vPosition

uniform vec2 uCanvasDims

void main(){

//get screen coord from frag coord

float screenX=(gl_FragCoord.x/uCanvasDims.x-0.5)*2;

float screenY=(gl_FragCoord.y/uCanvasDims.y-0.5)*2;

//find the difference between the fragments screen position and the points screen position then simply work out the point coord from the point size

vec2 pointCoord=vec2( (screenX-vPosition.x)/(vPointSize/uCanvasDims.x), (screenY-vPosition.y)/(vPointSize/uCanvasDims.x));

}

I've not tested so there maybe errors but hopefully you get the giest and it points you in a workable direction.

Paul