screentoworld zdepth problem

Hello I was hoping someone could help with a problem i've run into…

Im trying to draw in 3d. I'm using a wacom tablet, its X-Y values are converted into X-Y screen co-ordinates using jit.gl.render screentoworld message. The pressure is going to be the Z depth, this is where the problems arise.

Im using a patch I found long ago on this forum to move the camera around, to be precise the code lets the camera orbit 0,0,0. Using the code I can enter the camera amplitude i.e. how far the camera is from 0,0,0.

As i move the camera around I always want to be able to draw on the plane that bisects 0,0,0.

0,0,0 is always in the centre of the screen.

After reading the manual and lots of posts on here i thought that the below would hold true in regard to screentoworld's z depth component-

If-

camera amplitude = 100
near_clip = 1
far_clip =1000

Then

entering 0.1 (1000/100) into screentoworld's Z depth would allow me to draw on the plane that bisects 0,0,0.

However this is not the case. Screentoworlds Z-depth seems to not to operate linearly…

The below holds true

If CamAmplitude = 200 then Zdepth = 0.996

CA = 100        Zd = 0.991
CA = 50        Zd = 0.981
CA = 12.5        Zd = 0.961
CA = 6.25        Zd = 0.921
CA = 3.125    Zd = 0.681

Logically

CA = 1000    Zd = 1
CA = 1        Zd = 0

The above is true to three decimal places. As you can see every time CA halves Zd goes down by double what it went down before.

Now possibly this is an issue with the code I'm using to move the camera around or possibly its non-linearity in the Z component of screentoworld.

This has had me stumped for 1 and half days so it would be wonderful if someone could help.

Below is an example patch.

Thanks
Will

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*edit in this patch far_clip is at 1001 but it shouldnt throw the patch too far off!