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For a simple Texture map bump, which direction is positive and negative? If the image has some dark spots and light spots (a flagstone pattern) and I want the dark spots to look pushed down, what to i set the bump at?

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Bump mapping really doesn't work that way.  It's a trick that's done with the surface normal, it's not an elevation or displacement map.  Areas of the map which are the same color, either light or dark, will appear flat.  Areas which change color will appear perturbed.

Entonces... Como saber si emplear valores positivos y/o negativos??

The surface normal, a vector perpendicular to the surface on the "eye side", gets altered based on something called the gradient of the map.  Gradient, in the case of a 2 dimensional map, is a measure of the change of the map with respect to the x and y axes.  The sign of the normal reverses that perturbation in both x and y.  The actual effect it has depends on the geometry of the surface and the lighting.  There have been reports as well that I'm not doing this consistently for all of the engines-- I haven't checked the code on this so it might be true.

Gracias por tan tan pornta rspuesta. Segun he visto: en Path Trace, los valores negativos hacen ver como valles las partes oscuras de un mapa, en Hybrid sucede lo contratio.... Saludos

So the "darker areas" on this .jpg wouldn't make much of a difference between positive or negative bump setting? Is this a case were I need a monochrome 2nd image map to get a deep displacement effect?

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Best approach is trial and error.  Pick your engine and try a bumpmap.  Use a negative scaling if you want to reverse the way thing appears to pop-out or push-in.  You can often get close to what you're looking for.

Displacement maps can be very effective but they are costly in terms of calculation time and should probably be used sparingly.  They use the bitmap as height information-- dark areas are lower than light areas.  You can use color maps for these but it's often easier to control as gray scale.

i tried both ways and i couldn't actually tell from the rendering which way was better. I just made a displacement map for this pattern.

As a rule of thumb (because i don't think i understand it yet) when do you use a negative vrs. a positive bump?

There is no rule of thumb-- it's trial and error.  Use a negative number when it looks better.  Certain programs like Crazybump can help take some of the mystery out of it, but not all of it.  You're trying to somehow get gradient (or displacement) information out of a photograph-- it doesn't always work.

Te adjunto estos mapas, tal vez puedan servirte. El color azul es un mapa de normales. 

To put it short - black goes down and white goes up by positive bump parameter (eg. bump +0,5).

Is that what you've expected to find out?

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