How to derive the 3rd componenet in a normal map?

[991060]

This must be dumb, if I had x and y, I can say |z| = (1-x^2-y^2)^0.5, but how can I tell z's sign, I think it could be both + and - , right?

 

[Eronarn]

| | = abs value = ignore the normal sign. Ex, if it were -5, it would be 5. If it were 5, it would stay 5. If it were -| |, it would always be a negative.

Edit: stated it wrong in original post

The side that z is on is positive, z itself can be either

 

[jpaana]

In tangent space normal maps Z is practically always positive.

 

[Dio]

Yes - that's why this technique is usually used for tangent space maps.