Re: Tiff with signed samples

On 8/18/20 2:58 PM, Bob Friesenhahn wrote:
> With signed values, there might be a 'nominal' measurement value 
> equivalent to 0 with values then appearing above and below that 
> 'nominal' value and scaled to not lose anything important. Another way 
> to do things is to ignore negative values and only use the positive 
> values.
For Mars, elevation levels in meters are magically able to be stored 
comfortably in a 16 bit int without custom scaling, top to bottom it's 
about 31 km. In the nasa mars image, not quite sure what their 0 is 
physically, but the pixel values range -8200 to 21162. I'm not against 
flooding mars to ignore the negative values, but terraforming is a lot 
of work!
>
> Maybe they can't handle such a large image.  I am not exactly sure 
> what you mean by "create normal maps from heightmaps".  Are you able 
> to provide a direct reference to a description so I can see what you 
> are talking about?
>

Yeah, Gimp and Krita both freeze everything and fill up my swap memory. 
gm will at least resize it without doing that!

About normal maps, in Krita, there's an option Filter->Edge 
Detection->Height map to normal map, but it crashes for any image of any 
size I try. Gimp has something similar, but it's only in a newer version 
for which I need to upgrade my system, and I'm too lazy to do that at 
the moment. I've kind of wanted a command line alternative anyway. I 
could program it myself I guess, but why program when I can spend twice 
the time exhausting other options!

Basically, height map to normal map is this:
    http://cpetry.github.io/NormalMap-Online/
https://stackoverflow.com/questions/5281261/generating-a-normal-map-from-a-height-map

 From values of adjacent "height" pixels, compute a normal vector, and 
store as a unit vector XYZ in RGB channels. XY are tangent to the 3d 
surface and Z points away from the surface. 50% means 0. Default normal 
is sticking straight away from the surface, usually coded as (.5, .5, 1.0).

-Tom



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