Noisy Sensors and 16-bit Workflow

[fike]

I have been noticing noisy skies in some of my dramatic black and white conversions.  Last week I reprocessed some of these images (they were 24-image panos) using a 16-bit workflow.  The mottled skies were substantially improved.  

Does the increased noise in the more densely packed sensors make a 16-bit workflow more valuable?

Can the extra bit-depth of a 16-bit image ameliorate the negative impact of chroma noise?  

(I am not implying that 16-bit workflow reduces noise.)

I am suggesting that the small extra quality in color/light gradations may improve image quality enough to reduce the perception of noise.  

This image is one of the images where I saw some mottled sky effects that I thought could be improved with 16-bit workflow:
Trailpixie.net

[Jonathan Wienke]

What you're seeing is not "noise", it is posterization. And using an all-16-bit workflow will prevent or greatly reduce it. If it was really noise, it would still be there regardless of 8-bit or 16-bit workflow.

[fike]

What you're seeing is not "noise", it is posterization. And using an all-16-bit workflow will prevent or greatly reduce it. If it was really noise, it would still be there regardless of 8-bit or 16-bit workflow.

I understand and believe what you are saying, but I'm not accustomed to seeing posterization in a mottled look.  Normally it manifests itself across gradients.  though it isn't noise, it looks a bit like noise.  As a result, I wonder if low-level noise added to this sort of mottled posterization makes it even worse.

[ckimmerle]

I think the problem is that you are, in an attempt to give the sky drama, over-stretching the tonal range. The levels of gray in most skies is limited to just a couple of stops, so when you (we) try to expand that range into full-blown black to white it adds noise. As you have seen, it can be pretty bad in 8-bit images, and much better working 16-bit, but even that has it's limits.

You can greatly reduce the noise by working in RAW (if you're not) and creating two digital files: one standard and one toned dark for the sky. Use layer masks to overlay the darkened sky over the standard image. Using this method, you have a greater beginning tonal range.

Also, you can selectively add noise removal in only the sky area as it's primarily the darker and flatter areas that have the biggest problem.

Without seeing a full rez version as well as the RAW/original file, I can only guesstimate. However, I've witnessed those same issues from time to time (especially when trying to convert to grayscale using a heavy red filter), so I'm pretty sure that's at least part of the problem.

Chuck

[fike]

...(especially when trying to convert to grayscale using a heavy red filter)....

Chuck

yep, that's it.  On another version, I applied noise reduction to the skies with some success.  I do need to redo this image in 16-bit, but this is an HDR pano, so there are 33 images that need to be reprocessed, stitched, and HDR processed.  I need a couple of hours to work on this one.

[fike]

Here is a very quick redo (cropped to show the relevant area) done in 16-bit.  The same mottled texture is apparent.  It isn't caused by the red channel, more like a combination of the blue and cyan channels.  These were and are RAW images, converted to TIFF, stitched in PTGui and HDR done by exposure fusion.



While in color the problem isn't nearly as noticeable.  



Next thing I need to look at is my PTGui HDR work.  I have been using exposure fusion, and that could be causing the issue.

[jbrembat]

It isn't caused by the red channel, more like a combination of the blue and cyan channels.

Cyan channel does not exist and blue channel is clean.

Red channel is noisy. Green and blue channels are good.
You can denoise the red using a good denoiser, like PhotoDenoising.

In the color image, noise is not visible as you "see much more green channel".

Building black and white, you have to take that in acount. If you were able to reduce the red noisy you can use as much red you like, otherwise the red must be kept at minimum level.


Jacopo

[tokengirl]

fike,

What software are you using to make your B&W conversions?

[fike]

I am using PS CS4 for B&W conversion. I am using the channel mixer for the conversion.

As for the red channel, I had suspected that red wasn't the culprit because moving the red slider up and down in the B&W channel mixer had no effect on the apparent noise level.  Moving the blue and cyan sliders does increase the effect, particularly when either blue or cyan is adjusted more aggressively.  Adjusting them together seems to reduce the magnitude of the effect but not sufficiently.  

But, to ensure that I was right, I did Neat Image noise reduction to the red channel.  There was not apparent change in the pattern.  The noise pattern in this image is much smaller than these mottled blotches I'm looking at.

I don't think it is noise because the pattern is too large and noise reduction has minimal or no effect.
I don't think it is posterization because it remains after changing the workflow to all 16-bit.

I am using Exposure Fusion with PTGui, so that is my next are of investigation. I will try to make a real HDR image and tone map that image.  

Unless anyone else has other suggestions or insight.

[tokengirl]

I am using PS CS4 for B&W conversion. I am using the channel mixer for the conversion.

You might want to download the free trial of Nik's Silver Efex Pro and fool around with it a bit.  I find that I am much more able to control this type of noise in the skies using Silver Efex Pro as opposed to using CS4 or Lightroom for the conversion.  HINT:  Rather than applying a red filter in Silver Efex Pro (which can cause that same type of noise), choose Film Types>Sensitivity and reduce the blue (and maybe the cyan a bit too).

This software is not cheap if you decide to buy it, but I have found it to be worth every single penny.  I do a lot of B&W.

[jbrembat]

I don't think it is noise because the pattern is too large and noise reduction has minimal or no effect.

It is noise, on the red channel. The pattern is not too large.

Noise reduction have to be performed and viewed on red channel. In color image the red noise is about invisible (this is a "green channel clean" effect, as I said).

[attachment=18837:Red.jpg]

This is the red channel.

Jacopo

[fike]

It is noise, on the red channel. The pattern is not too large.

Noise reduction have to be performed and viewed on red channel. In color image the red noise is about invisible (this is a "green channel clean" effect, as I said).

[attachment=18837:Red.jpg]

This is the red channel.

Jacopo

I tried again to reduce noise on the red channel.  No luck.  I used the PS CS4 noise reduction filter and Neat Image noise reduction.  I did noise reduction on the combined RGB channel and on just the red channel.  The mottled pattern remains.  

I also made a standard HDR output from PTGui and tone mapped it with Photomatix and the noise remains. So it doesn't appear that the mottled appearance is caused by exposure fusion.

To summarize what I have tried:

• blurring sky with manual blending:  worked a little bit, but not very practical or complete

...so I wondered if there were more sophisticated approaches.

• 16-bit workflow to reduce posterization: didn't work
• noise reduction: didn't work
• noise reduction on red channel: didn't work
• change exposure fusion to HDR with Photomatix tone mapping: didn't work

One other note.  This image was taken at ISO 800 on a 50D.  Not an ideal situation, but the shadows are clean.  I would expect noise to show up there before it rears its ugly head in clear, blue skies.

[jbrembat]

noise reduction on red channel: didn't work

Incorrect. With a good denoiser, you can get a very good red channel.

Red channel after fast denoising using PhotoDenoising:

[attachment=18847:Red_denoised.jpg]

Jacopo

[fike]

Incorrect. With a good denoiser, you can get a very good red channel.

Red channel after fast denoising using PhotoDenoising:

[attachment=18847:Red_denoised.jpg]

Jacopo

Can you tell me a little bit more about your tools and procedure?  At first glance, your processing looks very good.

[tokengirl]

One other note.  This image was taken at ISO 800 on a 50D.  Not an ideal situation, but the shadows are clean.  I would expect noise to show up there before it rears its ugly head in clear, blue skies.

I experienced this problem sometimes with my old 40D, and from what I've seen, the 50D had even more noise problems with all those extra pixels crammed on the sensor.  Upgrading to the 5DMkII fully solved the problem though.  

[fike]

... Upgrading to the 5DMkII fully solved the problem though.

hmmmm...no.

[jbrembat]

Can you tell me a little bit more about your tools and procedure?  At first glance, your processing looks very good.

Simple, I used PhotoDenoising and performed the action only on red channel.
A demo of PhotoDenoising can be downloaded  here

Jacopo

[fike]

Simple, I used PhotoDenoising and performed the action only on red channel.
A demo of PhotoDenoising can be downloaded  here

Jacopo

I will try downloading it and give it a shot.  I wonder what PhotoDenoising is doing that Neat Image is not doing?

[jbrembat]

I wonder what PhotoDenoising is doing that Neat Image is not doing?

Why?

If you download PhotoResampling (from the same web site) you will find things that PhotoShop is unable to do.

Jacopo

[fike]

Why?

If you download PhotoResampling (from the same web site) you will find things that PhotoShop is unable to do.

Jacopo

I guess you are involved in the creation of this interesting program, so I my comments are going straight to the source.  Great!

I have downloaded the program, but it is unable to handle this large a file. It says "Insufficient memory for dib Denoise." The image's dimensions are approximately 12,000 7,000 pixels making a 343MB file.  the 16-bit was even bigger.  I have 8GB of RAM in my system. Will this software handle 16-bit?  I converted my image to 8-bit for working.  What is the max file size it supports?

I reduced the image size to 8,000 x 4,500 pixels with 8-bit color making a 140MB file and it runs now.  The "demo" text is annoying, but I guess this is a free trial.  How do I save?  Unless I am missing something, I don't see how to save and try the rest of my workflow in CS4.  

Your software looks very interesting and promising.  It has done what Neat Image couldn't do.  It needs much improved documentation, though. At $18 USD, it is almost in impulse-buy to try it out.  I wish I could save the file to inspect the results more thoroughly in the rest of my workflow.  It isn't as if all those 'demo' words across my image is going to let me use the image elsewhere. I am also disappointed that it can't handle larger images.  I do mostly panoramic images and 12,000 x 7,000 pixels or larger isn't uncommon for me.

thanks for the advice.