RE: [Algorithms] mach bands

[Akbar A. <sye...@ea...>]

>As an aside, are Mach bands anything to do with Ernst
>Mach, the speed-of-sound Mach number guy?
same guy.


peace.
akbar A.

you have "heard";
now read,
www.beconvinced.com


-----Original Message-----
From: gda...@li...
[mailto:gda...@li...]On Behalf Of Andrew
Jones
Sent: Thursday, October 05, 2000 4:21 AM
To: gda...@li...
Subject: Re: [Algorithms] mach bands



----- Original Message -----
From: Akbar A. <sye...@ea...>
To: <gda...@li...>
Sent: Thursday, October 05, 2000 2:20 AM
Subject: RE: [Algorithms] mach bands


>
>
> open up adobe photo, give it a high res..
> now do a simple gradient. from black to white.
> now look at it :)
> basically where ever you feel the color ramp is not right, that's a mach
> band.
> the problem with the coming to a white to fast is a "precision clamping"
> problem.

This isn't a Mach band. That's just a colour quantisation artifact.

"A phenomenon whereby piecewise linear intensity changes across a polygon
boundary trigger the human visual system into perceiving the boundary as a
bright band (3D Computer Graphics by Alan Watt, page 132). "
2nd-ary quote via Philip Taylor.

In laymans terms, Mach bands appear at the edges of shaded areas (i.e.
polygons),
when the colour gradient between adjacent polygons is different. When you
gouraud
shade polygons the RGB values are interpolated linearly. Even if the RGB
values are the
same and shared on the edge, there can be a sharp discontinuity in the
linear gouraud shading
gradient. Allthough this isn't intuitively a problem, the way you percieve
this makes the areas
immediately either side of the edge appear too bright and too dark.

If you want to try this in photoshop, don't do a single colour gradient. Do
two colour gradients which
have a matching edge, but which have different actual gradients. e.g. From
left to middle, blend
from RGB 0,0,0 to 64,64,64 and then from 64,64,64 to 255,255,255. The centre
line where
the gradient changes should show a Mach band.

As an aside, are Mach bands anything to do with Ernst Mach, the
speed-of-sound Mach number guy?

Andrew

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