View Full Version : Weird gradient issues...

Michael Neborak
05-24-2009, 05:46 AM
I have been noticing this weird gradient issue when I export from REDrushes (or any other debayering program) to a prores 422 file (but it happens with all the other files as well except if I choose to use no codec when exporting).

The two examples are below:

This is the original (a big file as its not encoded in any way):

And this is the one transcoded into a Prores 422 file:
http://mivid.net/tmp/prores422.png (http://mivid.net/tmp/prores422.png)

Notice how the color seems to flow into each other in the original, but jumps in the prores. It looks like the prores is only exporting a limited amount of colors...

Also on a sidenote, the color is faded and off (gamma?) in all exports except the none codec...

What is going on? I would appreciate any help!


05-24-2009, 08:22 AM
This has to do with the bit-depth you are exporting into. RED footage is 12 bit so exporting the footage to, say 8 bit, will give you harsher gradients like you are showing in your example because the lower bits means less colors being used. For example, 8 bit has 256 colors and 16 bit has 65,536 colors. I would suggest a higher quality codec if you really want to get a better gradient but I believe Prores 422 is 10 bit... which is still high quality.

Raul Gonzo
05-24-2009, 08:48 AM
Hey Michael, I have found that adding a little bit of noise, or a very slight gaussian blur (masked off in the problematic area) helps with that sort of banding.

05-24-2009, 09:01 AM
Very true! You could also try to soften the image (similar to Raul suggestion of a gaussian blur). It would make the gradient seem much smoother.

Jim Hoffman
05-24-2009, 09:18 AM
This issue drives me crazy too. I have a very difficult time getting out smooth images in many shots. B&W.... ugh. Prores 422 not the easiest to work with when it comes to gradients.

Uli Plank
05-24-2009, 11:30 AM
For critical footage you should work in uncompressed or DPX log. ProRes is not bad, but it's 10 bit an compressed.

Michael Neborak
05-24-2009, 12:27 PM
Thanks for all the replies! I knew it had to do with the bit depth, but it is unfortunate that there is no solid workaround, just ways to mask it. However, the slight blur idea is a very good one and I will have to try it.

Best Regards,

Dan Hudgins
05-24-2009, 01:04 PM
Thats why I am using uncompressed 48bpp TIF I/O, even DPX will show banding and posterization if you do extreem grading, the extra two bits helps.

Dithering does not necessarily help if you are going to compression something like MPEG-2 (H.262) later since the grain will stand still for several video fields. I made my dithering on output to 24bpp BMP frames optional for that reason. Otherwise the DVDs can have blocks of "grain" that stand still then blink as fuzz.

Dan Hudgins
05-25-2009, 01:18 AM
I was thinking that I should mention another issue in banding, you should not sharpen images that have undergone bit reduction.

If you take a 12bit image and reduce it to 8bits per primary, then sharpen the boundry between tone bands is then exaggerated, that happens because sharpen makes the tone difference at a boundry between two tones greater than one tone step, so you then get a light and dark line along the tone bands. If there is noise or dithering in the edge of the tone bands you then get rings around the noise spots at the edge of the tone bands making the boundry between the tone bands even more obvious.

When you go from 12bits to 10bits this can be less, but the more you sharpen, the larger the zig-zag that is added along the tone boundries. Its like this:

Before sharpen:

Line 400: 120
Line 401: 120
Line 402: 120
Line 403: 121
Line 404: 121
Line 405: 121

After sharpen:

Line 400: 120
Line 401: 119
Line 402: 118
Line 403: 123
Line 404: 122
Line 405: 121

So you see that rather than a rise of one tone step at the boundry, you now get a rise of five! That sharpened rise is now closer to 6bits rather than 8bits as far as tone smoothness.

If you blur after the sharpen has been introduced you can reduce the step, but you get more ringing so its best not to have any sharpen of reduced bit depth images at all.

Adaptive sharpen can help, since it will not sharpen low contrast edges, but you can get a video look that way, since video equipment uses that kind of sharpen to reduce noise/blur pickup in video data.

Virgil Kastrup
05-27-2009, 07:38 AM
This is a serious issue on all redline quicktime renders(All i've ever made anyways). Even when rendering uncompressed quicktimes. If I need full debayer quicktimes for online I usually use REDCINE or render out tiff's from redline and use quicktime pro to render uncompressed quicktimes.