Channel Booleans question

I am trying to create a mask which is related to the y position of a simple 3d scene - a rotating cube.
I can see the y position information from the render node and as a first step I used a Channel Boolean with r g and b all set to Y position fg. This gives me as expected a black and white image shaded correctly to represent the y value of the scene. However I cannot understand the RGB values which are shown.
The y values range from about 3 to 19 but the RGB values range from 127 to 129 if in 8 bit which is a very limited range.
In a very similar example comp that I downloaded from the forum, the RGB values were almost identical to the Y values.
I have tried using different image resolutions but I cannot get it to respond as I would have expected.
I am a nubie using free fusion 7.7. Have I got the wrong idea about how it should work?
Thanks
John Epton

You want a mask for the Y position? Or for the cube itself?

The Y position can come from a Locator. The cube can render it's own mask.

Have you tried setting the renderer to float16? The position channels should always be float by default but maybe the channel boolean does weird stuff.

Thanks all.
I switched everything to 16 bit float and it seems to have resolved my problems. I now get sensible RGB values out of the Channel Booleans tool even if some are out of range. these I dealt with using a low gain value in a Brightness/Contrast tool before using a Bitmap Mask tool to operate on the final Merge.