It looks complicated. I always think if I don't clip shadow and highlight
even my image file is sRGB there will be no lost of gamut. aRGB is more
important for 8 bit editing for 16 bit there should be no problem for both
sRGB and aRGB, no?
BTW, for your example of Green from a value of 64 change to 256 (255?), it
should be much more than 2 stops. With DPP I change the exposure from -1 to
+1 stop, the change of value is from 80 to 178. With E-1 file and Olympus
Master 2 it is 67 to 156.
C.H.Ling
----- Original Message -----
From: "Ken Norton"
> >
>> If you're serious on this (I assume you are), then I must start
>> thinking this whole thing over.
>>
>
> Not really that big of a deal. aRGB is a better EDITING and STORAGE
> colorspace than sRGB and presents a wider color gamut to an output device
> which supports a wider colorspace than sRGB.
>
> What happens here with colorspaces is that sRGB spreads the steps out
> evenly
> across the entire dynamic range from max black to 100% lumenance of each
> color channel. aRGB robs tonal steps from part of the tonal range and
> applies those steps in other parts of the tonal range. When you convert an
> aRGB image back to sRGB, it remaps those steps to the nearest step in the
> sRGB colorspace. The selected gamma also determines the spacing of those
> steps to lumenance values, so the spacings aren't equally stepped across
> the
> spectrum.
>
> Let's take a RAW file. For simplicity sake, let's say that the A-D
> conversion is 8-bit. This gives us 256 brightness levels (steps) per color
> (0-255). Unfortunately if the sensor is a linear device and the supporting
> analog circuitry is not shaping the response curves prior to A-D
> conversion,
> then one half of those brightness levels are assigned to the top stop
> (zone
> from here on out) of dynamic range. This means that we have 128 steps
> assigned to JUST one zone. But then the next zone down has 1/2 of the
> remaining bits or 64 possible steps. The third stop or zone of brightness
> range has 32 possible steps, the next one has 16 possible steps and so on.
> By the time you get to the darkest zones, you literally have three zones
> represented by seven steps. This is the "solarization" of which I speak.
>
> To maximize image quality because of this A-D conversion, we talk about
> "Expose to the Right" which pushes the maximum amount of the image as far
> up
> as we can so we have the most number of steps in the effective range. But
> this usually ends up overexposing the image. No big deal, though, because
> during conversion and placement into a colorspace, we'll reassign all
> those
> high bits down to where we really want them to be. As long as the camera
> isn't doing any curves adjusting PRIOR to the A-D conversion or in-camera
> prior to the RAW file writing everything should be OK. (Note the premise
> of
> that last statement--most, if not all cameras do adjust the image prior to
> digitization and will require a different ICC profile to get things to map
> correctly).
>
> When converting the RAW data into editable data, we remap all those steps
> to
> the sRGB colorspace which pretty much, other than the gamma offset,
> equally
> assigns steps from black to max, or to aRGB which puts more steps in part
> of
> the tonal range than others. By stealing bits from, say, the middle tones,
> we are able to assign more bits to the highest and lowest values.
>
> While we are at it, we also have the choice of 16-bits (two bytes) per
> color
> or 8-bits (one byte) per color when we convert and save. Working in 16-bit
> mode makes editing a whole lot easier because you don't get solarization
> artifacts when making major adjustments.
>
> When we convert between colorspaces, we have the choice of preserving
> colors, saturation or high/low thresholds, but we can't save them all, so
> we
> decide based on personal experience, preference, the image itself and
> output
> medium which one of these traits we wish to preserve.
>
> I mentioned a day or two ago how we can end up with part of the image
> being
> represented by only a few bits of data. Let's assume our original image
> was
> four stops under exposed. That means that the entire image dynamic range
> could be represented by no more than 32 possible tonal steps!!! During
> conversion and editing we pull up the exposure which now spreads those 32
> original steps across 256 steps (8-bit) or 65536 steps (16-bit). Just
> because we spread those 32 original steps out to 65536 steps doesn't mean
> that it has 65536 steps worth of tonal values--it still is just the
> original
> 32 steps with a whole lot of gaps. Now, let's apply some additional
> curves,
> color and constrast adjustments to this image file. Every time you do this
> you are pushing values up or down and potentially eliminating original
> values. This is why we can end up with a portion of the image getting only
> a
> handful of original steps preserved through the image editing process. Of
> my
> original 32 steps, if the dynamic range of the picture is four stops, and
> the steps are equally spaced, then each stop has only 8 steps. Depending
> on
> where in the dynamic range we're looking, how the colors are mapped, the
> original A-D conversion, etc., etc., you can end up with far fewer than 8
> steps per stop or zone. That's 3-bits.
>
> Before anybody thinks that we don't frequently do 4-stops of adjustments,
> I
> challenge this by a simple test. Import a RAW image file with no
> adjustments
> other than exposure offset. Keep the colors straight and don't apply any
> curves adjustment as the ONLY adjustments made prior to the assignment to
> colorspace is exposure compensation and highlight recovery. (Note, there
> may
> be some variances on this from one converter to another, but generally
> speaking all curves and color adjustments are made after the RAW file has
> been remapped into a 16-bit working colorspace). The file should only have
> a
> max black and a max white if the picture is of a high contrast scene. But
> the colors should be pretty flat. Work in 8-bit mode for this, as it's
> easier to see the effect. Use your "inspector" and look at the RGB values
> for given pixels on the image. Now, start editing the picture, changing
> the
> curves, saturation, etc. Now use your inspector to look at the RGB values
> at
> those exact same pixels again. It is NOT unusual for you to have adjusted
> parts of the image between two and four stops. The overall brightness of a
> spot may not look different to the human eye, but the RGB values will
> reflect a change. If the values change from "32,64,128" to "32,256,128"
> the
> green channel for that pixel has increased two stops. Overall, the image
> may
> not have changed drastically, but each pixel may have. Usually, this just
> shows itself as an increase in noise, but on an expanse of a single color,
> it will show up as solarization.
>
> Too much information?
>
> AG
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