Using the word "wrong" is a little harsh. It's a very useful way to
look at computer graphics, having coded both OpenGL applications and
image processing applications, I'd say it's a fundamental concept to
computer graphics. Halfdan's was scientific and probably
mathematically correct tho, but a little less applicable to using
floats with XSI, IMO, however I'm not an engineer.
--
stockholm.postproduction | www.stopost.se
On 2/10/06, Andy Jones <andy(at)thefront.com> wrote:
> There are some right answers and some wrong answers here. Halfdan's
> answer is right. Arvid's is wrong. Greg's is wrong. While it's true
> that a 32-bit floating point value has 2^32 possible values, it's not
> correct to think of these values as shades of grey between 0 and 1.
> That would be more like a 32-bit integer which also exists, but isn't
> typically used in computer graphics. The purpose of a floating point
> representation is to store values in a much wider range than 0 to 1.
> For example, the position data for vertices in your scenes is stored in
> a floating point representation. Floating point numbers are typically
> represented with the 32-bit "float" type or the 64-bit "double" type.
> For images, there is also a 16-bit floating point type being popularized
> for image data via OpenEXR and graphics hardware. This type is called
> "half," presumably in honor of Halfdan Ingvarsson. It's useful because
> for images, we're typically working in a range larger than [0,1], but
> not as big as what we need with general purpose floating point data
> types. And the level of precision required is less because the levels
> stored are pretty much always approximated anyway.
>
> There are many reasons you might choose a floating point representation
> for your image data. Greg is correct that 16 bit integer precision
> typically alleviates banding problems that arise when doing standard 0-1
> compositing operations, like what you do in a typical AfterEffects <7.0
> workflow. In fact, 8 bits is basically enough to encode images for
> viewing, once no additional color space modifactions are to be made.
> That's why the output buffers in computer graphics cards have pretty
> much stuck at 8 bits per channel (remember when there were fewer?). The
> eye can't typically differentiate the shades of gray beyond 8 bits or
> 256 shades. Maybe one or two more bits wouldn't kill us, but 16 is way
> more than enough and is needed only if parts of the range are going to
> be stretched.
>
> As it concerns the linear workflow we've been talking about, floating
> point representation is useful because instead of attempting to render
> image data that is ready for viewing on a computer monitor (and
> therefore will be clipped to [0,1] anyway, we're attempting to store
> relative amounts of light which don't have to stop at 1. Once we have
> the images representing amounts of light, we can perform operations such
> as exposure and gamma correction to prepare the data for viewing on a
> computer monitor. At that point, the data can be clipped to [0,1] and
> integer representations are useful again. In general, output formats
> can be a lot more restrictive in terms of range and precision than
> intermediate data that's going to be modified in some way. That's why
> while 10-bit Cineon has been great for film plates and you can't really
> tell what's been digitized to cineon and reprinted, there is still a
> push in the film industry to use formats like OpenEXR for storing frame
> data.
>
> It's really all about representing data with consistent precision levels
> inside and outside of a [0,1] range.
>
> -Andy
>
> Sylvain Moreau wrote:
>
> > >>>So isn't there a limit to how many colors we can actually
> > perceive? I'm not sure if my eyes are 32-bit compliant.
> >
> > Good question, here is part of the answer:
> >
> > - An average person can perceive around 100 000 colors. Trained
> > professional can see between 200 000 to 300 000.
> > - One man out of eleven has some sort of color blindness and actually
> > sees a lot less than 100 000 colors (color blindness is very rare
> > among women)
> > - 8 bits RGB gives you more than 16 000 000 colors but this is not
> > enough because computers use linear interpolation and your eye
> > perception is not linear (the eye sees a lot of subtle changes in the
> > dark but very little in the bright area)
> > - 16 bits is enough. My take on this is your eye will stop perceiving
> > changes between colors somewhere in the 10 to 12 bits range.
> > - 32 bits is overkill but still very useful in some situations (e.g.
> > heavy color correction, hdr, etc.)
> >
> > syl
> > ~~~~~~~~~
> > wotomoro.com
> >
> > ----- Original Message -----
> > *From:* Meng-Yang Lu <mailto:ntmonkey(at)gmail.com>
> > *To:* XSI(at)Softimage.COM <mailto:XSI(at)Softimage.COM>
> > *Sent:* Friday, February 10, 2006 2:29 PM
> > *Subject:* Re: embarrasingly stupid question
> >
> > So isn't there a limit to how many colors we can actually
> > perceive? I'm not sure if my eyes are 32-bit compliant.
> >
> > -Lu
> >
> >
> > On 2/10/06, *Todd Akita* <takita(at)earthlink.net
> > <mailto:takita(at)earthlink.net>> wrote:
> >
> > Man, that is a LOT of crayons.
> >
> > -T
> >
> > Greg Smith wrote:
> >
> > > a floating point digital image is a 32-bit image. with
> > 8-bit images
> > > each RGB channel has 256 levels of value, a 16 bit image has
> > 65,536
> > > levels of value per channel and a floating point, or 32 bit
> > image has
> > > 4,294,967,296 levels of value per channel. take that number
> > to the
> > > power of 3 and you get how many possible colors you get in
> > RGB space,
> > > which is a lot of color.
> > >
> > > Greg
> >
> >
> >
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> >
> >
>
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