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question re: greyscale

question re: greyscale

2015-04-12 by paulmwhiting@...

Hello,

Here's a Dropbox link to the 21-step greyfile supplied with QTR:

https://www.dropbox.com/s/7gszzza0mcpod99/21step.tif?dl=0

I wan to measure the density of each step. I note there are three areas to this scale. The top area is a continuous scale, from dark to light. The second, or middle, area shows the steps but with a sort of "scallop" to each patch. The bottom area shows the steps separated by thin white lines, and the steps are more uniform than than the second area's steps.

It seems to be me I should be measuring density from the third, bottom, area to get a more representative reading. Is that correct? If so, just wondering why that middle area has "scalloped" steps. You may have to enlarge the view to better see what I mean.

TIA,

Paul



Re: [QuadtoneRIP] question re: greyscale

2015-04-12 by Paul Roark

The patches that appear "scalloped" are not. That is your eye/brain masking being made more apparent to you/us. (It's cool and tells us something about our visual system.)

Pull the test strip into PS and measure the density with the eyedropper set to 1 pixel to convince yourself.

Paul
www.PaulRoark.com
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On Sun, Apr 12, 2015 at 9:22 AM, paulmwhiting@... [QuadtoneRIP] <QuadtoneRIP@yahoogroups.com> wrote:

Hello,

Here's a Dropbox link to the 21-step greyfile supplied with QTR:

https://www.dropbox.com/s/7gszzza0mcpod99/21step.tif?dl=0

I wan to measure the density of each step. I note there are three areas to this scale. The top area is a continuous scale, from dark to light. The second, or middle, area shows the steps but with a sort of "scallop" to each patch. The bottom area shows the steps separated by thin white lines, and the steps are more uniform than than the second area's steps.

It seems to be me I should be measuring density from the third, bottom, area to get a more representative reading. Is that correct? If so, just wondering why that middle area has "scalloped" steps. You may have to enlarge the view to better see what I mean.

TIA,

Paul




Re: [QuadtoneRIP] question re: greyscale

2015-04-12 by Myron Gochnauer

If you load this into Photoshop you can use the “Info” panel to check the values (“K” eyedropper set to greyscale and 8 or 16 bit).

You will discover that only the top row is a smooth scale from black to white. The second and third rows make uniform steps as indicated by the dividing lines. The apparent change in density within each step is a perceptual illusion, not something “objectively” measurable.

It is really hard to believe that what you are seeing is an illusion. I keep going back and putting the eyedropper on the “light” side of a zone and then carefully moving it to the “dark” side. . . and yes, the reading a precisely the same. Good grief!

Myron




On Apr 12, 2015, at 1:22 PM, QuadtoneRIP@yahoogroups.com wrote:

Hello,

Here's a Dropbox link to the 21-step greyfile supplied with QTR:

https://www.dropbox.com/s/7gszzza0mcpod99/21step.tif?dl=0

I wan to measure the density of each step. I note there are three areas to this scale. The top area is a continuous scale, from dark to light. The second, or middle, area shows the steps but with a sort of "scallop" to each patch. The bottom area shows the steps separated by thin white lines, and the steps are more uniform than than the second area's steps.

It seems to be me I should be measuring density from the third, bottom, area to get a more representative reading. Is that correct? If so, just wondering why that middle area has "scalloped" steps. You may have to enlarge the view to better see what I mean.

TIA,

Paul






Re: [QuadtoneRIP] question re: greyscale

2015-04-12 by Roy Harrington

Yes. Its just an optical illusion. But I also notice that the file you have here shows
some evidence of JPG compression -- little halos around sharp edges.
On the Mac distribution its a .psd file and does not exhibit this artifact. I don't
have the PC distribution immediately available for checking.

Roy

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On Sun, Apr 12, 2015 at 9:32 AM, Paul Roark roark.paul@... [QuadtoneRIP] <QuadtoneRIP@yahoogroups.com> wrote:


The patches that appear "scalloped" are not. That is your eye/brain masking being made more apparent to you/us. (It's cool and tells us something about our visual system.)

Pull the test strip into PS and measure the density with the eyedropper set to 1 pixel to convince yourself.

Paul
www.PaulRoark.com

On Sun, Apr 12, 2015 at 9:22 AM, paulmwhiting@... [QuadtoneRIP] <QuadtoneRIP@yahoogroups.com> wrote:

Hello,

Here's a Dropbox link to the 21-step greyfile supplied with QTR:

https://www.dropbox.com/s/7gszzza0mcpod99/21step.tif?dl=0

I wan to measure the density of each step. I note there are three areas to this scale. The top area is a continuous scale, from dark to light. The second, or middle, area shows the steps but with a sort of "scallop" to each patch. The bottom area shows the steps separated by thin white lines, and the steps are more uniform than than the second area's steps.

It seems to be me I should be measuring density from the third, bottom, area to get a more representative reading. Is that correct? If so, just wondering why that middle area has "scalloped" steps. You may have to enlarge the view to better see what I mean.

TIA,

Paul









--
Roy Harrington
roy@...
www.harrington.com

Re: [QuadtoneRIP] question re: greyscale

2015-04-12 by Keith Schreiber

Paul,

That is an optical illusion due to the adjacent darker and lighter steps. You can verify this for yourself in Ps by setting a section of the info panel to display K % only, and moving around any step with the Color Sampler (Eyedropper) tool with sample size set to point or 3x3 or 5x5 pixels. They really are solid blocks of even tone.

Keith


Re: question re: greyscale

2015-04-12 by paulmwhiting@...

I was sure I posted a thank you several hours ago but I don't see it now.

Anyway, thanks all... that is truly surprising. I've used this scale many times but always wondered about this. I did in fact pull it up in PS and sure enough, the eyedropper measured the same throughout the step. Keith's answer helps get at the "why". And Roy: the thing is, that was a tiff file I put in the Dropbox - go figure!

Cheers,

Paul

Re: question re: greyscale

2015-04-12 by paulmwhiting@...

PS: I suspect the white separation lines in the third row help to minimize the adjacency effect Keith mentions... but the "scallops" are still slightly visible.

Re: [QuadtoneRIP] question re: greyscale

2015-04-13 by Roy Harrington

Hi Paul

I did see that it was a tiff but who knows how the file actually was created. I know I've seen a jpg version somewhere so I'd assume it came from there.
Step wedges are hard to make perfect. One thing you may find curious is: in Photoshop select a few steps and in levels greatly increase the contrast. Then you can see the subtle details.
Most of this is too small to make a practical difference.

Roy
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On Sunday, April 12, 2015, paulmwhiting@... [QuadtoneRIP] <QuadtoneRIP@yahoogroups.com> wrote:


PS: I suspect the white separation lines in the third row help to minimize the adjacency effect Keith mentions... but the "scallops" are still slightly visible.



--
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Re: question re: greyscale

2015-04-13 by paulmwhiting@...

Thanks Roy,

I take your point about that tiff originating as a jpg. Now that I think of it, I sometimes first save a photo in jpg as a tiff, and then edit that... if I quit, save and come back later, I don't lose quality as you do when you work on a jpg.

I did try selecting a few steps, increasing the contrast and then enlarging it. I first noticed that the continuous tone first row is not continuous at all... it consists of very narrow but discrete steps. And I think I spotted the halo effect at the edges of the 21 step row you mentioned.

Regards,

Paul
http://www.paulwhitingphotography.com

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