Different types of normal maps?

NephilimFree

Why do some normal maps in 3D engines differ in color?

For example:

In one normal map the "neutral" color appears to be grey (#7F7F7F), but in another blue (#7F7FFF).

Alex_J

there is different coordinate systems. you can google "left hand versus right hand coordinate system" and search the wiki here it has a good explanation.

the way it is packed might also be a factor. it is possible to pack normal to two channels only and use a math function to rebuild the missing channel. That way an additional grayscale map can be used without additional texture file needed.

ben cloward on youtube has a video showing how to do that in unreal (and maybe unity as well)

EarthQuake

Yeah, the first image probably shows a two-channel (red green) normal map. There are some compression formats that provide better quality for normal maps (higher bit-depth) without making the file size too big. The blue channel is reconstructed in the shader.

NephilimFree

Thanks for the response.

After researching handedness in normal map coordinate system I only found that the order of the colors changes in a right vs left hand system:

So I did not know about Channel Packing before but it seems likely this could be the case for the red/green/grey normal maps.

Is there a standard list of commonly used methods of channel packing or a way to identify how it's used based on a normal map image?

Alex_J

not that i know of. i think that it always has to be the up axis which is derived from the two other axis but somebody more tech saavy than me could say otherwise.

if you are viewing the textures like ripped from some game the naming convention might provide a clue.

edit: didn't see earthquakes response above. thats the expert, not me!

NephilimFree

It appears the normals in this engine are in DDS "BC5_SNORM" compression format.

Now I just need to find a way to convert between a standard normal map and this format.

Klunk

The blue channel is reconstructed in the shader.

In this case I don't think even that's needed, it's just assumed the blue channel is white (set the blue channel to white in the top image and you get much the same output as the second image) with tangent space normal maps there's never really a lot of detail in the blue channel. All the normal perturbation is in the red and green channels, the blue is really just changing it's overall length.

if you imagine each pixel is a tiny plane with a lookat constraint to a point directly above it then the red channel moves the point  +- in x and green +- in y and blue +- in z.
blue will only really change the plane orientation ( the lighting) when x and or y are relatively large (or very negative) otherwise it's effect is pretty subtle.

in reality the blue channel saves you from having to normalize the normal vector in the pixel shader but if you are willing to live with some over perturbed normals you  can do without....
white on the left, white and normalized on the right, and with textures

though normal.z = normal.x  + normal.y; gives a bold effect...

if the lights in the right direction

NephilimFree

Klunk said:

The blue channel is reconstructed in the shader.

In this case I don't think even that's needed, it's just assumed the blue channel is white (set the blue channel to white in the top image and you get much the same output as the second image)

This appears to be correct after testing. As EarthQuake mentioned, it's a red/green only compression format "BC5_SNORM".

Making the blue channel white makes all of the grey/red/green images look like standard normal maps.

Thanks all