# scaling point sprites with distance

How can you scale a point sprite by its distance from the camera?

GLSL fragment shader: `gl_PointSize = size / gl_Position.w;` seems along the right tracks; for any given scene all sprites seem nicely scaled by distance. Is this correct?

How do you compute the proper scaling for my vertex attribute `size`? I want each sprite to be scaled by the modelview matrix.

I had played with arbitrary values and it seems that `size` is the radius in pixels at the camera, and is not in modelview scale.

I've also tried:

``````gl_Position = pMatrix * mvMatrix * vec4(vertex,1.0);
vec4 v2 = pMatrix * mvMatrix * vec4(vertex.x,vertex.y+0.5*size,vertex.z,1.0);
gl_PointSize = length(gl_Position.xyz-v2.xyz) * gl_Position.w;
``````

But this makes the sprites be bigger in the distance, rather than smaller:

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To answer my own question, here's what I got working:

The scaling in the GLSL vertex shader is:

``````gl_PointSize = (heightOfNearPlane * pointSize) / gl_Position.w;
``````

Where you compute your `heightOfNearPlane` using the viewport height and the field-of-view angle you constructed the perspective matrix with:

``````float fovy = 60; // degrees
int viewport[4];
glGetIntegerv(GL_VIEWPORT,viewport);
float heightOfNearPlane = (float)abs(viewport[3]-viewport[1]) /
(2*tan(0.5*fovy*PI/180.0));
``````

Which you pass in as a uniform.

Thx to the coders on irc.freenode.net ##opengl that helped me sort this out!

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Don't forget to call `glEnable(GL_VERTEX_PROGRAM_POINT_SIZE)`to allow setting of point size from vertex shader. –  Christoph Halang Dec 17 '13 at 11:28

I found the solution to scale PointSprites:

``````const float SIZE_MULTIPLIER = 300.0;
attribute float inSize;
uniform vec2 g_Resolution;
…

vec4 modelSpacePos = vec4(inPosition, 1.0);
vec4 position = g_WorldViewProjectionMatrix * modelSpacePos;
#ifdef POINT_SPRITE
//  vec4 worldPos = modelSpacePos  * modelSpacePos;
float spriteDist;

if (position.w == 0.0) {
spriteDist = 0.00001;
} else {
spriteDist = position.w;
}

gl_PointSize = (((inSize * SIZE_MULTIPLIER * (g_Resolution.x/g_Resolution.y)) / spriteDist) * (g_Resolution.x/g_Resolution.y));
#endif
``````

Here is my talk on jMonkey engine: http://hub.jmonkeyengine.org/forum/topic/pointsprote-size-limitation/#post-250817

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