Hi,
In a book I'm studying this formula is written:

0 < r, g, b, a < 1

Where beneath the < a underscore is written.
Which I believe simply means less or equal then.

In practical sense I know that r, g, b and a should all be in the range of 0 - 1.

What I'd like to understand is why it's written out this way (because other formulas are also written out like this).

The formula

0 ? r, g, b, a ? 1

is an abbreviated version of these four formulas:

0 ? r ? 1
0 ? g ? 1
0 ? b ? 1
0 ? a ? 1

In turn this means
0 ? r
r ? 1
0 ? g
g ? 1
0 ? b
b ? 1
0 ? a
a ? 1

In English: r, g, b and a are numbers between 0 and 1.


EDIT: This might help: http://en.wikipedia.org/wiki/Inequality_%28mathematics%29

Thanks, makes much more sense now.
To be honest 0 < r confuses me a bit, but not when I turn it around: r > 0

To be honest 0 < r confuses me a bit, but not when I turn it around: r > 0

Try to picture 0 and r on a number line:

If a particular number is farther to the right on the number line than is another number, then the first number is greater than the second (equivalently, the second is less than the first).

This is quite a missleading formula, you could read it as "...variable r greater than zero,variable g, variable b and variable a lesser than one ...".

Quite a missusage of comma operator, more definitive expression would be:

r,g,b,a ? (0,1) ? .. (provide some numbers set)

r,g,b,a ? (0,1) ? .. (provide some numbers set)

Considering that (0,1) is typically an open range (excluding 0 and 1) and a closed range is more typically expressed with [0,1] this is no less confusing ;)

r,g,b,a ? (0,1) ? .. (provide some numbers set)

Considering that (0,1) is typically an open range (excluding 0 and 1) and a closed range is more typically expressed with [0,1] this is no less confusing ;)

The orginal formula contains >, what means "greater than", I think excluding interval is in place then

The orginal formula contains >, what means "greater than", I think excluding interval is in place then

Where beneath the < a underscore is written.
Which I believe simply means less or equal then.

Which I believe simply means less or equal then.

there is >, and >=, written of course differently as a glyph, he used simple >, what means close any, but never equal.

Which I believe simply means less or equal then.

there is >, and >=, written of course differently as a glyph, he used simple >, what means close any, but never equal.

Your reading comprehension is beyond repair. Just carefully read the original post until everything makes sense.