"Dennis Miller" <dhm### [at] comcastnet> wrote in message 
news:43f11f93$1@news.povray.org...
> Thanks Warp. Is there any way to dig a little deeper? How does the image 
> map become the pigment function? Assuming there are RGB values for every 
> pixel in my image, what exactly gets mapped to the pigment function? Is 
> there any scaling of parameters values, or an offset? What would those 
> values look like? I would like to get a better understanding of how this 
> process works, as I am clear that the function comes from the bitmap. But 
> I'm trying to get a better handle on how that happens...
> And regarding the grayscale values, again, are those quantized to some 
> range of values? What's the process here as well?
> thanks much.
> Dennis

The image_map gets mapped from <0,0> to <1,1> in the x-y plane regardless of 
the image size and dimensions and is projected infinitely along the z axis. 
Without the 'once' keyword, the image is repeated inon the x-y plane at 
every unit square.  This gets converted to a function by assigning every 
x,y,z point a value equal to the colour of the image as mapped to the x,y,z 
space.   Next, an isosurface evaluates a given function.  The surface of the 
isosurface appears when the function is equal to the threshold limit 
(default is 0).  So when you use just the Sphere function, the surface shows 
up where the function equals 0, which (depending on how your Sphere function 
is defined...) gives you a spherical surface.  Now when you add the image 
function to the sphere function, the grey component (which will range from 0 
to 1 as evaluated from the standard conversion of rgb values to greyscale*: 
0.3R + 0.59G + 0.11B) of the image function as created by the image map for 
each point in the x,y,z space is added to the evaluation of the Sphere 
function at each each point.  The surface of the 2 functions added together 
appears where the value of this combined function is equal to 0.

Read "3.4.1.5.1  Specifying an Image Map" in the help for more info on image 
maps and try the isosurface tutorial "2.3.3.3  Isosurface Object" to 
understand better how isosurfaces are evaluated and how the functions 
defining themcan be manipulated.

-tgq

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