Re: On ProPhotoRGB
Re: On ProPhotoRGB
- Subject: Re: On ProPhotoRGB
- From: email@hidden
- Date: 26 Feb 2006 10:20:02 -0000
- Encoding: 8bit
> a) What are the meaning and consequences of ProPhotoRGB straying into areas
> outside the spectrum locus?
Draw a circle on a piece of paper, then draw a triangle that encompasses every single point along that circle's radius within its perimeter. You'll notice that in order to bring the entire circle into bounds, you have to push the triangle's verticies out beyond the circle's edge. The same thing happens when you try to define a color space with only three primaries on the CIE diagram: the spectrum loci is curved, while the triangle you get by connecting the three primaries together is not. A triangular peg isn't ever going to fit a circular hole perfectly, so if you want to get something on the edge into bounds, you need to push those bounds places they probably should not go, such as the realm of imaginary colors. You could also define additional primaries, but putting such a model into use would be impractical at our present level of display technology (I did read something about somebody somewhere putting together a monitor that used 6 or so primaries recently, so maybe so!
meday).
Some of the colors that can be captured on traditional film lie along the edge of the spectrum loci, so if ProPhoto wants to be able to record them, it needs to push its green and blue primaries out a bit into la-la land.
The consequences are that you end up wasting a portion of your tonal range on colors that cannot be seen, recorded or reproduced in any way, leaving you with fewer real colors to work with despite the visibly more expansive gamut. This occurs only because of the finite nature of digital information storage, and not because of any natural phenomenon. When you can place any one of 256 possible values in a single pixel and no more, throwing away any portion of those possible values on colors that cannot be seen effectively wastes them, and posterization can result. This is why it is generally recommended that you work with 16-bit images when using wide gamut working spaces, because then you have 65536 possible values to assign to every pixel, and can afford to throw a handful away if necessary without introducing any objectionably abrupt tonal transitions. If none of the colors in your image lie in any of the additional areas ProPhoto covers, then there is no reason to use it.!
The debate seems to arise amongst photographers who for some reason or another have convinced themselves that these colors are rarely encountered in the "real world" no matter how many times you show them areas of bright greens and/or blues clipping in Adobe RGB. There was once something of a negative stigma associated with 16-bit images since Photoshop was severely crippled when it came to working with them, but that is no longer the case, and the only good reason not to is the increased resource consumption.
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