Great Trevor. Thanks much. I'll have a look at the relevant docs.
Thanks again for the explanation.
Best.
Dennis

"Trevor G Quayle" <Tin### [at] hotmailcom> wrote in message 
news:43f148d8$1@news.povray.org...
>
> "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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