How to decrease alpha value of pixel the farther out you go?

I thought this would be easy...

This is a 3D volume of a 1 x 1 x 1 dimensions cube.


This is the 3D Volume, regular view....it's not too transparent, so it's hard to see "inside" of it.


This is a slice of the 3D Volume, slices are about .05 units thick
It's a slice about mid way through the volume....so you see there are structures



Here is my "alpha-sized" 3D volume. The result is not what I thought it would be.
Plus there is a cube corners artifact going on

 

Here is what I do in the ray-stepping function...at the end of the function...

float lenRay = 1 - length(prev_ray_pos - .5);
ray_col.a = lenRay;
ray_col = clamp(ray_col, 0, 1);
return ray_col;

When I do this, the third picture is the result I get....
What I reasoned was, that the farther out the ray vector is, the more transparent it should be, thus the "1 - length(prev_ray_pos - .5)

So the larger the vector, the more transparent it should get....

Well, the results show that this approach does not seem to work.

I know, it's a hard question, maybe somebody might have an idea of the best approach to go...

 

I suspect you're thinking about this the wrong way. Really you should be applying some kind of scale and / or curve to each sample's opacity. Basically find where the tex3D() call is and multiply the results by 0.5.

 

will try, thx

 

Better results, the best value is actually a very low one, .05