Photoshop: Tones not Uniform ?

[Ken Lee]

I have made some grey-scale step wedges, and notice that as I move the mouse over the different steps, I see different brightness values within the same region.

Is this a common occurrence ? I would think that if PS designates a region as, say 50% brightness, or RGB 122, 122, 122, then the entire region would be the same value.

Perhaps someone could try this and tell me if you get the same results.

I am using CS3 on a Mac Powerbook G4.

It's hard to calibrate something against a step wedge, if the step-wedge itself is non-uniform.

[Colin Graham]

I had this problem making a 101 step tablet and ended up doing it in long hand; i.e. making a grid and using paintbucket to fill each one with the correct K% and this gives perfectly smooth steps. I had tried the gradient fill method with adjust>posterize or the pixelate>mosaic filter and got results like yours. If you'd like a copy of the high res version don't hesitate to ask.

[Ken Lee]

Colin -

On Windows + Photoshop CS, my JPG step wedge appears entirely uniform within each step. One or two of the values are off by 1 point, but I get the same value wherever I move the mouse within a step.

On the same platform, looking at your step wedge, I also get perfectly uniform values along the top row, but the values themselves are surprising: Starting at the left, they are as follows: 0, 1, 2, 4, 6, 8, 9, 11, 12, 14. If I view the same image on my Mac with CS3, the values are not uniform: they vary widely inside each step.

Attached is a simple tablet with Brightness values of 0, 25, 50, 75, and 100. On saving to JPG, the 0 changed to 1. I made it on Windows, with Photoshop CS. If I view that same file on the Mac with CS3, the values in each step appear to vary.

I hope that this isn't a Mac problem, but rather a PS problem. Maybe we have found a long-standing bug. Either the values are variable, and we have just learned about it, or the values are uniform, and either CS3 or Mac can detect the difference.

I would love to know which which, is which.


Here

[Marko]

Ken,

I think your problem is in the output format - jpg - rather than Windows.

Jpeg is a lossy, compressed format suitable for images with lots of shades and continual transitions such as gradients.

Gif, on the other hand, is a losless format suitable for large, uniform areas of solid color.

As a rule of thumb, it is best to use jpeg for photos and gifs for illustrations and diagrams.

I hope this helps.

[Brian Ellis]

If you don't get a satisfactory answer here you might post your question to the Yahoo group "digitalblackandwhite:theprint." There are questions and anwers there all the time about using step wedges for linearizing and other purposes.

[Marko]

One more thing - The best way to save it as a gif would be to use Save for Web and then pick gif - 256 colors - no dither. This should give you perfectly uniform values for up to 256 shades of gray.

[Ken Lee]

"I think your problem is in the output format - jpg - rather than Windows"

Thanks Marko, I thought about that too, but the non-uniform values are apparent before the images are saved.

Just start with a greyscale image, select a region, and "pour" some value into it with the paint bucket tool. Move the mouse around, and you will see (on Mac + CS3) that the values fluctuate considerably. Try the same procedure on Windows + CS, and the region appears uniform. Don't save anything.

[Ken Lee]

"you might post your question to the Yahoo group "digitalblackandwhite:theprint."

Thanks Brian.

After a quick glance, it seems that many people on that site, use the step wedges which ship with the Quadtone Rip. Those are in PSD format.

I had a look at them on my Mac + CS3 setup. The values fluctuate, just as they do when I look at my own step wedges. On my Windows setup, they are perfectly uniform.

So again, I wonder: is it something inherent on the Mac, or in CS3 ?

[Marko]

Ken,

I just tried several methods on CS3/Mac and was not able to reproduce your problem. I tried the following:

1. Black to White gradient edge to edge followed by Posterize/21 steps
2. Rectangular selection then paint bucket
3. Rectangular selection then Option/Delete

All three methods resulted in perfectly even patches.

May I suggest you try again and make detailed notes so we can try to replicate the problem? If it is a bug, it would be useful to find out and report it, and if the problem is entirely yours, then you will end up solving it...

[Ken Lee]

Wonderful -

I will do that. I have to run out now for a few hours, but will do so as soon as I can. Note that Colin says he had the same problem. Perhaps he can explain his method too.

Many many many thanks !