Digital Artform

It's a good idea to break scenes up into 5 or so value levels, from black to white, and to try to determine with accuracy which portion of a painting falls into which value level.

Be careful when turning color images into black and white images using Photoshop. The software may not act as you expect. In particular, it will place the grayscale divisions in even steps, which may not suit your purposes. Don't rely on this technique in your painting practice; it's more a way of explaining an idea. Develop your own skills in determining what areas of your image deserve which values.

Some bright colors appear to be brighter to the human eye than other bright colors. Blues tend to look darkest. The common equation for converting RGB to Luminosity is the NTSC (National Television Standards Committee) formula:

luminosity = 0.299 x red + 0.587 x green + 0.114 x blue

If you go through the trouble of changing the image mode to grayscale, you will get what you expect you should get.

If you do not use the mode > grayscale technique, you will get surprising results.

If you merely desaturate the image, which is what most people do, you will get a mathematically plausible but psychologically wrong looking image in which all equally saturated colors become equal grays - regardless of their hues, or their effect on human perception.

Unexpected? As a painter, you will probably want to avoid this outcome.

UPDATE 5/26/05

Craig Mullins (screen name spooge demon, in this thread) is the master of this. Here's an excerpt from what Craig Mullins says on the matter of values:

"Decide what is in light and what is in shadow and don’t mix them up. Think like a comic artist. Two values, but if they are well thought out and designed and drawn they can look totally real. Think like that, but instead of making the light white and the shadow black, make the light a 7 and the shadow a 3. Then go ahead and use 5-10 in the light and 1-3 in the shadow to pull out sub forms. DO NOT use 1-5 in any part of the light, or use 5-10 in any areas of the dark. Keep you edges a little softer in the shadows, a little sharper in the light, you are done. (0 is black, 10 is white) Deciding what is in shadow and light for a particular object is pretty hard in words. I will leave that up to you and that is 99 percent of the struggle."

I try to learn from him.

UPDATE 12/10/05

I was re-reading Paul Haeberli's classic Grafica Obscura today and noticed that he makes the point that the usual NTSC luminosity weights are intended for gamma 2.2 work, not linear gamma 1.0 work. Here's an excerpt...

http://www.sgi.com/misc/grafica/matrix/index.html

Converting to Luminance
To convert a color image into a black and white image, this matrix is used:

float mat[4][4] = {
rwgt, rwgt, rwgt, 0.0,
gwgt, gwgt, gwgt, 0.0,
bwgt, bwgt, bwgt, 0.0,
0.0, 0.0, 0.0, 1.0,
};

Where rwgt is 0.3086, gwgt is 0.6094, and bwgt is 0.0820. This is the luminance vector. Notice here that we do not use the standard NTSC weights of 0.299, 0.587, and 0.114. The NTSC weights are only applicable to RGB colors in a gamma 2.2 color space. For linear RGB colors the values above are better.

In effect, this calculates:

tr = r*rwgt + g*gwgt + b*bwgt;
tg = r*rwgt + g*gwgt + b*bwgt;
tb = r*rwgt + g*gwgt + b*bwgt;