[Radiance-general] colorpict and materials

[Jack de Valpine]

Hi Lars,

I believe that what you need to do is take your colorpict modifier and 
apply it to a material such as a plastic that uses the reflectance of 
your material sample as the rgb parameters. I believe that you DO want 
to use macbethcal to callibrate the color values of the image and 
determine an estimated reflectance. Then using normpat on the image will 
move the values in image so the average is 1.0.

So steps as follows:

   1. acquire sample material image by photography, scan....
   2. callibrate sample image with macbethcal
   3. use callibrated sample image to estimate average color
   4. calculate reflectance based on average color values, grey(r,g,b)
   5. normpat the callibrated image

Then create material as follows:

void colorpict color.image.pattern
7 red green blue <normpat picture>.pic picture.cal Px Py
0
1 <aspect>

color.image.pattern plastic color.image.material
0
0
5 <grey(r,g,b)> <grey(r,g,b)> <grey(r,g,b)> <s> <r>

Since the colorpict is normalized to an average value of one, it will 
modify the plastic around the reflectance of the plastic. For example, 
if colorpict returns 1.1 for red then the red of the plastic will be 10% 
brighter.

In short I think that there are two main ways to use colorpict

   1. final reflectance set by colorpict image - colorpict is used to
      set final reflectance of material, that is use callibrated image
      colorpict modifier and material with reflectance of  1.0
   2. final reflectance set by base material - colorpict is used to
      modify reflectance set by material, that is normpat image
      colorpict modifier is used to modify reflectance set by material

I hope this is helpful.

Regards,

-Jack

Lars O. Grobe wrote:

> Hi,
>
> sorry, so again, I try to be clear in asking this time ;-) By the way, 
> have nice holidays (I hope that's the reason for your travel ;-)!
>
> All is about the question how to map pictures onto a surface without 
> corrupting its material properties. I understand that colorpict 
> multiplies the material color components with those of the picture.
>
> If I have a grayscale image processed by normpat (which means that the 
> average gray value is 1.0) and apply it to a material, the overall 
> color and brightness will remain the same. So I get the "pattern" from 
> the image, but the surface still has the correct material properties 
> (color, brightness etc). I used this so far.
>
> Now I want to use a colored picture for mapping. I also apply normpat 
> to it, so, as far as I understand, the average of all R, G and B must 
> be 1.0, right? For example, I use a normpat'ed picture of green 
> marble. I than apply this using colorpict to a surface, which has a 
> "marble" material. Will the overall color and brightness still be that 
> of the defined material, as the picture map has the average of 1.0?
>
> The background: I try to use exact data for material definitions, but 
> the image maps can't all be color corrected. So I want the overall 
> color and brightness from defined materials, e.g. from the plastic 
> material, and use the map only for what I would call "local color 
> variation". The reason is that I have e.g. red marble, got its color, 
> brightness and all that defined as plastic marble. But the marble has 
> blue particles which won't appear if I use a grayscale imagemap. So I 
> want to use a normpat'ed (NOT colorcorrected e.g. by macbethcal!) to 
> bring these blue parts onto the surface. The whole surface however 
> must still have the average color of my plastic marble material.
>
> If I understand the man-page of normpat, that's just what it was 
> invented for. However, I am a bit unsure, as all Radiance 
> documentation uses colorpict with a bright white material.
>
> TIA+CU, Lars.
> -- 
> Lars O. Grobe
> grobe at gmx.net
>
>
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> Radiance-general at radiance-online.org
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>

-- 
#	John E. de Valpine
#	president
#
#	visarc incorporated
#	http://www.visarc.com
#
#	channeling technology for superior design and construction

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