Calculating distance from vert to specified point

OpenGL
1

Let’s say I have a point, called MyPoint, that I pass into the vertex shader:

uniform vec3 MyShader;

In my vertex shader I want to calculate the distance from MyPoint to the curent vertex. How do I do this?

Thanks,
CB

2

First you must ensure that your vertex MyPoint is in the same coordinate space as gl_Vertex. Then use the built in function distance(myPoint,gl_Vertex).

Edit: I must write that your vertex MyPoint and gl_Vertex are both in the same coordinate space.

3

What coordinate system are gl_Vertex in?

4

Any space under the sun. It’s just a vec4, after all. Heck you could pass in quaternions if you had a mind to…

5

You can use something like this:


uniform vec4 MyPoint
...
//Transform vertices in eye coordinates
vec4 eyeVertexPos = gl_ModelViewMatrix * gl_Vertex;
vec4 eyeMyPointPos = gl_ModelViewMatrix * MyPoint; 
float dist = distance(eyeMyPointPos,eyeVertexPos);

I assume here that MyPoint is defined in object (modeling) coordinates like gl_Vertex is.

6

Wow, you are doing two additional matrix multiplications, just to get the distance? Guys, you are really complicated…

uniform vec4 MyPoint;
…
float dist = distance(MyPoint,gl_Vertex);

This is enough. If you do not know what coordinate-spaces are, then you are most certainly always working in world-space, so that no transformations are necessary.

Jan.

7

Wow, you are doing two additional matrix multiplications, just to get the distance? Guys, you are really complicated

I submit this sample code because if there is a scale factor in the model view matrix, this should affect the distance between the two points.

8

A valid concern, but the point uniform should be given in the space it will be used.

9

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