Technical Image Data Question

M
Posted By
mp
Oct 18, 2005
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335
Replies
5
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Closed
If i have a 16-bit image that’s very dark and shadowy and the image is being rendered in 8-bit. (monitor)

If I make any levels adjustments, does the data that’s being thrown out when the image is rendered for the monitor "re-appear" or otherwise factor back into the image I see on the monitor?
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MW
Mike Warren
Oct 18, 2005
mp wrote:
If i have a 16-bit image that’s very dark and shadowy and the image is being rendered in 8-bit. (monitor)

If I make any levels adjustments, does the data that’s being thrown out when the image is rendered for the monitor "re-appear" or otherwise factor back into the image I see on the monitor?

Black is still black and white is still white. The only difference is the number of steps between.

So, to answer your question: Not really, but it depends on exactly what changes you make.

—
Mike Warren
My web gallery: http://web.aanet.com.au/miwa/mike
M
mp
Oct 19, 2005
Black is still black and white is still white. The only difference is the number of steps between.

A16-bit image: 2^16 = 65536 shades of gray. Pretend my image really does contain 1000 shades of gray and I make some levels adjustments.

I tend to believe PS is calculating changes on 8 bits per-pixel color because of the speed at which it works is the same regardless of the bit-depth. It may support high-bit images, but the actual editing calculations appear to be equivalent to working in 8-bit.

Can someone enlighten me? Does anyone have any evidence proving otherwise?
MW
Mike Warren
Oct 19, 2005
mp wrote:
Black is still black and white is still white. The only difference is the number of steps between.

A16-bit image: 2^16 = 65536 shades of gray. Pretend my image really does contain 1000 shades of gray and I make some levels adjustments.
I tend to believe PS is calculating changes on 8 bits per-pixel color because of the speed at which it works is the same regardless of the bit-depth. It may support high-bit images, but the actual editing calculations appear to be equivalent to working in 8-bit.
Can someone enlighten me? Does anyone have any evidence proving otherwise?

If you start with an 8 bit file and convert it to 16 bit then there are still
the same 256 steps between black and white.

In this case any level shifting will produce near enough the same as if you were doing it in 8 bits.

If, however, you were to do editing that creates new data (eg. a gradient or blur) then the new data will have extra levels which will be available for further editing.

That’s why I said it depends on what you do.

—
Mike Warren
My web gallery: http://web.aanet.com.au/miwa/mike
MW
Mike Warren
Oct 19, 2005
mp wrote:
I tend to believe PS is calculating changes on 8 bits per-pixel color because of the speed at which it works is the same regardless of the bit-depth.

On this point: PS (like all 32 bit programs) performs calculations on 32 bit data not 8 or 16 bit. That’s why there is no speed difference.

—
Mike Warren
My web gallery: http://web.aanet.com.au/miwa/mike
MR
Mike Russell
Oct 19, 2005
A16-bit image: 2^16 = 65536 shades of gray. Pretend my image really does contain 1000 shades of gray and I make some levels adjustments.

In general, depending on how steep your levels adjustment is, your final image will also contain 1000 shades of gray.

I tend to believe PS is calculating changes on 8 bits per-pixel color because of the speed at which it works is the same regardless of the bit-depth. It may support high-bit images, but the actual editing calculations appear to be equivalent to working in 8-bit.

The levels interface shows 8 bit curve control points, but the final result is high-bit. Photoshop does this by effectively interpolating between the 8 bit values to perform the appropriate hibit calculation.

Can someone enlighten me? Does anyone have any evidence proving otherwise?

In CS2 you can use the info palette to display hex values to prove this, for example by using levels to change a gradient with values from 0 to 1000. Otherwise you can use a hex dump utility to verify that the values are distinct, or apply extreme curves and look for posterization. —

Mike Russell
www.curvemeister.com

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