GH1 mud problem - suggestions

A

Aki_Hartikainen

Carbonite Member
I will start a new thread about the infamous GH1 mud problem as the problem is better understood and methods to fight the mud are being developed. The mud problem is the aggressive noise in low light scenes causing posterization. It is caused by bug in GH1 RGB to YUV encoder, which adds noise to the UV color channels. The noise is amplified by the lower spatial resolution of the color difference channels. They are the 2:2 part of the 4:2:2 sampling. Once decoded back to RGB, the noise resides mostly in the lower resolution Red and Blue channels. Thankfully the Green channel is of rather good quality in GH1.

More about it in this thread: http://www.dvxuser.com/V6/showthrea...ot-with-a-GH1-in-low-no-light-and-NOT-GET-MUD.

Here is a GH1 low light shot (ISO 400). The mud noise patterns are clearly visible especially in the red seat:

blogmudoriginal.png


Here is the Red channel of the image:

blogmudredchannel.png


Here is the Green channel of the image:

blogmudgreenchannel.png


You could see that the Green channel is cleaner than the Red channel. To fight the mud in low light means that the emphasis has to be on the Green channel in post production after the color grading, like this:

blogmudgraded.png


When the Green channel is boosted and/or Red and Blue channels are lowered, the final image becomes cleaner. Obviously the solution can not always be to turn the colors to green for the final image, so the color grading has to start already while shooting low light scenes. It appears both Red and Blue channels are almost equally affected by the mud noise, so one potential solution is to use a purple filter either in front of the lens or in front of the light sources to boost Red and Blue channels while filming. When you can later lower the intensity of the Red and Blue channels in post production, it should work a bit like noise reduction. The purple filter makes it possible to shift the emphasis on the cleaner green channel while grading.

I would suggest to first light and white balance for neutral white balance during filming the scene, then add purple filter for the lens or light sources and open up lens iris or increase light wattage to compensate loss of brightness caused by the purple filter.

It is not a one button solution for the mud problem, but atleast could help to start experimenting to achieve better quality low light scenes with GH1.

Aki Hartikainen, FourSeasonsHD.com
 
Here is another example of a muddy GH1 original material turned into less muddy by boosting the Green channel while grading:

blogmud11.png


Enlarged view shows that the Green channel is better:

blogmud12.png


Next step is to test some camera and lighting settings to see if it possible to boost Green channel and reduce Red and Blue channels while grading and land with desired colors and less mud. Here is a link to the full size image, if anybody is interested: http://www.fourseasonshd.com/blogmud13large.png
 
Here is a small grading test in very low light (ISO 400). More testing is needed to find the lighting and grading methods that only barely mask the GH1 noise patterns.

 
I have some methods and examples available to fight GH1 mud, if anybody is interested. Here is an image (ISO 400):

blogGrading.png


The mud patterns are clearly visible in the original image, especially between the lenses there are vertical colored stripes that plague low light scenes easily. In the graded version the mud is lesser. Here is the link to full size image:

http://www.fourseasonshd.com/blogGradingLarge.png

Most of the mud resides in the Red and Blue channels between values 50 - 90. Green channel made from gray scale image combined with G255 Color Matte is of high quality. Below are screen grabs how to create a clean Green channel and flatten the mud in Red and Blue channels using curves. Finally I created a new nested sequence and added Add Grain filter with Eastman ISO 100 grain preset.

Red Channel:

blogGradingRed.png


New Green channel is made by combining Gray scale image plus G255 Color Matte on track 4:

blogGradingGreen.png


And Blue channel:

blogGradingBlue.png


Finally created a new nested sequence and used curves some more and added Add Grain filter with Eastman 100 grain preset. The results are promising but may render really slow. Link to short video:

http://www.youtube.com/watch?v=Jv1sbrj-G94
 
The amazing mud reduction for GH1 is any shooting technique that forces one to either boost Green channel in post production or lower Red and Blue to get desired colors.

This means purple filter in front of the lens. It would reduce Green channel, so one has to amplify Green channel in the post. To maintain brightness would mean opening up iris while shooting, which would record Red and Blue channels amplified and use more bits for color as well. When the mud values in Red and Blue channel are constant, bringing the level of purple down in post should reduce mud.

Now the question is, where to get a suitable purple filter for testing.

blogMudRemoval1.png
 
Some quick test shot through a Blue filter CCB4 1.5x. Graded with Three Way RGB color filter and Premiere Noise HLS added as the last step. Some mud reduction is already present. Hues fade to shadows better. A keying method is needed to fade the image to gray scale below certain luma values. Human vision can not see colors very well in shadows but there may be color noise in the image of these cameras which could be eliminated by blending to gray scale for the shadows.

blogGrading2.png


Link to full scale image http://www.fourseasonshd.com/blogGrading2large.png
 
Makes sense that green would be cleaner when shooting on a camera with 4:2:0.

I'd think that would be expected, since half of the pixels only have red or blue data (and not the other), blue is generally noisy because it is harder for the sensor to detect, and each of these colors is interpolated at various points...

Came across this, looks interesting: http://phyxware.com/Cleaner.html
 
GH1 has rather good Green and Gray scale channels and rather bad Red and Blue channels. A method has to be developed that forces the camera to allocate more values and bits to Red and Blue channels to improve Chroma. The link you provided has also a product called Chroma Reconstructor, which might do something useful also.

This same applies also to GH2 but obviously it has better Chroma than GH1. GH2 has some mud too, just not nearly as much as GH1.
 
Last edited:
Okay, here is something that can be tested, as you could see from the images on top the stripes seem to be horizontal for Red channel and vertical for Blue channel.

Divide the footage into separate channels as I did in above posts, add Vertical Gaussian blur for the Red channel and Horizontal Gaussian blur for the Blue channel. The combination will still be posterized but add filter Noise HLS Auto in Premiere Pro with very low settings as the last step to create animating grain. Needless to mention GH2 would require only fraction of these effects.

blogGrading3.png


Same footage as above. Link to full size image: http://www.fourseasonshd.com/blogGrading3large.png
 
I have proof now that using suitable color filters on the lens can improve quality. For this video I had a blue filter on the lens (not in the one outdoor scene):


The camera would have been able to compensate for the blue filter in-camera, in which case the hues would have looked more normal. But the quality improvement is still there, because the blue channel in the camera has seen more action than usual. This is a sort of noise reduction, the blue and red channels don't have the same quality as the green channel, so boosting the poorer channels in-camera allows later pulling them down. As you can see the shadows are cleaner than they would have been had I tried to grade the material to look like that without the lens filter.

It appears suitable purple filters are very hard to come by. The purple filter would put more action on both red and blue channel in-camera thus allowing noise reduction for both of the problematic channels. I have ordered purple filters from China, but unfortunately they have not arrived. More testing will commence once I get suitable filters. They should not be too strong to allow normal white balance later in post.
 
And here are the results from all of the mud reduction methods combined:


- Blue filter on the lens
- GOP-1 settings on the GH1
- AVCHD transcoded into Cineform intermediate codec
- Resized to 1440 * 816 for chroma interpolation
- Added channel blur for red channel
- Levels flattened to 16 - 235
- Colors adjusted with levels and curves
- Unsharp mask
- Resized back to 1080p
- Added 0.008 Gaussian noise in Sony Vegas
- Saved into Photo JPEG codec for upload

Next step is the get a bit more serious about the gamma curves, so that lens filtered video could be turned into normal colors (but with noise reduction). Ideally ready made templates and presets would be created to allow most of the steps added in a bit more productive manner.
 
You must log in or register to reply here.
Back
Top