Gradient Tool

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Marpel
Posts: 692
Joined: September 13th, 2009, 3:19 pm
What is the make/model of your primary camera?: Nikon D810
Location: Port Coquitlam, British Columbia

Gradient Tool

Post by Marpel »

I have fabricated a vertical gradient that I am trying to use as a mask to blend two solid colour images (each 6000 x 4000 pixels in size). The gradient is solid (black) for the bottom 2/3rds then gradually goes to white at the top. The problem that occurs is both the mask and the resultant image show obvious banding-like horizontal "striations" rather than a smooth gradation. In fact if one looks closely, it appears the striations lighten a bit near the edges of each striation then goes dark again, then light, then dark etc as it progresses from dark to light. And, of course, this issue is more obvious if the two solid colours are farther apart so the contrast is greater. I tried to smooth these striations through use of a Gaussian Blur (even tried repeated blurs) on both the mask as well as the resultant image, however this did not help. I have used the gradient tool to do similar operations in the past without this problem and it would appear (only guessing from memory of those previous operations) that the "banding" shows more prominently the more the actual gradation portion narrows. At least I have not noticed it when the gradient runs the full image.

For those times when the gradient is fairly narrow, is there a means to smooth out the gradient so these striations are not obvious??

Thanks,

Marv
den
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Re: Gradient Tool

Post by den »

It is not natural for photo realism to have such large solid pixel areas for both the color images and the Black/White Gradient filter image... ...try adding 2 to 3 percent Gaussian noise to the Gradient filter image... ...then Composite-Blend.

Illustration: a partial vertical Gradient filter image comparison at 3:1 zoom with the approxiate change in gradient as described in the OP... ...right half is shown with an added 2.5% gaussian noise...
capture_02122015_092034.jpg
capture_02122015_092034.jpg (36.04 KiB) Viewed 7285 times
At normal resolutions and viewing distances the striations/noise most likely will not be noticeable...

...den...
Dieter Mayr
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Re: Gradient Tool

Post by Dieter Mayr »

Marv,

Did you try 16 bit for the gradient and solid colour image ?
It should be much better using 16 bit.
It could also be that the banding is just virtual, meaning that your monitor simply can not display the gradient correctly.
I had a laptop a while ago that produced sigificant banding in the blue tones, but when viewing the image on a real monitor all was ok.,
Maybe you have the chance to see your image on a different monitor ?

Just some thoughts, hope it helps.
Dieter Mayr
Marpel
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Re: Gradient Tool

Post by Marpel »

Den and Dieter,

Thanks for your replies.

Den, I tried a small amount (actually tried a couple different amounts) of Gaussian noise as you suggested, however, it makes little difference in the mask as the banding is still visible.

Dieter, the two solid colour images which are to be subject of the blending are both 16 bit, however, I am quite sure PWP only accepts 8 bit traditional masks and gradient-generated masks. In fact, I just checked the Gradient Tool Dialogue and could not find an option for a 16 bit result. I am using an NEC monitor which I calibrate on a regular basis. It is certainly not the highest-end monitor on the market but is of relatively good quality. Having said that, I do not have a second monitor at hand to compare. And funny you mentioned that your laptop had issues with the colour blue as the colours I am trying to gradient-blend are dark blue and light blue. I don't think it is a blue colour issue as the banding is quite evident in the black and white gradient as well.

I do not know if this issue is only evident on a monitor and would be fine in print, however, I need to be sure prior to sending it off (I send off a digital file and the end user does the printing and would hate for the problem to manifest itself in print as well).

Anyway, thanks again for your replies.

If anyone has any other suggestions, I would be interested in hearing them.

Regards,

Marv
Marpel
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Re: Gradient Tool

Post by Marpel »

Out of interest, I attempted to duplicate what I have been trying to do in PWP, with the gradient tool in CS6.

This tool allows a solid colour gradient to be generated without the need for the initial steps (generate a mask first then composite a couple solid colour images, using that mask) which PWP requires. I used the same colours that I used in PWP. The resultant image appears to also display the same horizontal banding or striations that PWP does and, in fact, to my eyes these striations appear more obvious in the CS6 image, although not by very much.

So, it is either both PWP and CS6 are unable to do a gradient without obvious striations (on the screen at least) or, as Dieter suggested, perhaps the problem does lay with my monitor, although as previously stated, I don't have a second quality monitor to compare. Other than my prior comment about the reasonable quality of my monitor, I also note that when I zoom into these images, the striations also enlarge with the image. Would this not suggest the striations are image based and not monitor based? Or am I wrong in this respect?

Regards,

Marv
Charles2
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Re: Gradient Tool

Post by Charles2 »

I made a similar gradient. Viewed on a NEC PA monitor calibrated to Adobe RGB color space, the striations are visible at 1:1.
Dieter Mayr
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Re: Gradient Tool

Post by Dieter Mayr »

Marv,

Didn't think of it, of course you can't use 16 bit images as a mask. Sorry.
But as you mentioned that you created a gradient in PS directly with the colors, you can do that in PWP too.
In the Gradient window, you have that Black to White line with 1 and 2 at the beginning resp. the end.
Click the 1 or 2 and you get a Colorpicker to chose the colors for the gradient, with shift click you add additional points, with ctrl click you remove them.
So, if it goes for gradients with solid colors you can build a lot directly.
And in 16 bit, too.
Dieter Mayr
Marpel
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Re: Gradient Tool

Post by Marpel »

Charles,

Good to know that this is visible by others on an NEC. Thanks.

Dieter,

I've probably done dozens of these type of gradients without having a clue that PWP could do a colour gradient straight from the tool. Funny enough, as I was using CS6 to do that, I actually thought to myself "Why the heck doesn't PWP have this capability?" Little did I know!! Thanks very much.

Regards,

Marv
Dieter Mayr
Posts: 453
Joined: April 24th, 2009, 11:47 am
What is the make/model of your primary camera?: Nikon D700
Location: Salzburg / Austria

Re: Gradient Tool

Post by Dieter Mayr »

Marv,

Did you have a look at the histogram of your gradient ?
A gradient over the whole width or hight on the image should give a perfect rectangle, for lengths that are not divisible by 256 for 8 bit color there are some periodic spikes, because not all colors can have the same count of pixels.
But it all looks regular.
If your gradient starts at about 1/3rd of the image hight you have of course a bigger count of the dark pixels, but the gradient portion should look regular, too.
Dieter Mayr
jsachs
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Joined: January 22nd, 2009, 11:03 pm

Re: Gradient Tool

Post by jsachs »

Sounds like small irregularities in your monitor profile. Try setting your monitor profile to sRGB and see if the banding goes away.
Jonathan Sachs
Digital Light & Color
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