RE: [Algorithms] Gamma correction an HDR images?

[<c.s...@ph...>]

Technically a HDR texture is a collection of numbers with enough =
precision and range beyond the 0..1 range.
Whether it is in gamma space or linear space, only the content creator =
knows.

Display cards for PC's have a history of outputting voltages that are =
proportional to RGB values.
Due to the fabric of the CRT circuitry the resultant light power gets in =
effect raised to a power near 2.

So greyscale 128 is about 1/4 the intensity of greyscale 255.

You can veryfy this by comparing a checkerboard pattern with a solid =
greyscale of 181, which corresponds to sqrt(1/2).
They should be ~about~ equal.

The gamma phenomenon introduces the convenience of allocating more =
precision to the dark colors for precision limited 8-bit RGB.
Since the darkest grey representable in 8 bit RGB is not 1/255, but =
really it is ~ 1/65025, which would need 16-bit precision in linear =
space. So gamma is also a form of compression. You will get ugly banding =
if you try to output an 8-bit precision framebuffer "linearly".

All our infrastructure today (Photoshop, digital cameras, scanners, etc) =
evolved around exchange of information in gamma-space, so we have to =
deal with it.

The bit-exact gamma value is not so much important (is different from =
device to device anyway) as it is more to remember is that gamma is in =
the first order a power of 2, which also makes things easy to calculate. =
Linear conversion before summing pixels is done by ^2 and re-gamma after =
summing pixels is done by sqrt().


-----Original Message-----
From: gda...@li... =
[mailto:gda...@li...] On Behalf Of =
Bra...@Pl...
Sent: Wednesday, June 01, 2005 8:00 PM
To: gda...@li...
Subject: [Algorithms] Gamma correction an HDR images?


So I'm continuing work on our texture pipeline, doing gamma corrected=20
(linear space) box filtering to generate mip maps, and everything is=20
grand.  However, now there's the issue of HDR images, and whether or not =

to gamma correct them before mip mapping.  Now, I've always been under =
the=20
impression that HDR images are already in linear space and don't need to =

be gamma corrected.  However, how does the graphics card "know" that it =
is=20
in linear space?  For example, is an HDR texture that only contains =
pixels=20
in the range of 0.0 to 1.0 (an LDR texture that happens to have an HDR=20
pixel format) somehow treated differently than a regular LDR texture?  I =

feel like I'm overlooking some vital piece of information here.

So, should I or should I not gamma-correct HDR images before mipping?

Thanks, and sorry if I munged the terminology in posing the question.  =
My=20
understanding of linear vs. gamma space and related things is still=20
somewhat cursory.

Brad...



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