Quantcast VirtualDub histogram, blacks never go red? - digitalFAQ Forum
  #1  
06-05-2021, 10:40 PM
kingbean kingbean is offline
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Hello,

Recently I've been utilising VirtualDub's histogram during my captures, and have been generally quite satisfied with the results, compared to just leaving my brightness setting at the default.

However, on some tapes it seems that lowering brightness to remove illegal white values results in a rather dark video. The black end of my histogram never goes into the red.

In the VirtualDub capture settings guide, the example is given as follows:

Quote:
Originally Posted by sanlyn View Post
Histogram...
The luma levels histogram works only in preview mode and not during capture. The histogram displays a graphic view of input luma levels for checking if the input signal lies within the preferred video range of y=16-235. The histogram displays 16-235 data as blue data across the graphic horizontally: invalid values beyond 16-235 display as red in the side margins (left=darks, right=brights). Red indicates crushed darks and/or clipped whites during capture. If you see a flood of red at either side of the graph, the signal contains luma values that clip during capture. Some slight spillover is allowed now and then. Remember that black borders or bottom-border head-switching noise affects the histogram. These can be temporarily masked with the "Cropping..." dialog (see next post) during capture setup .

Histogram display (video extending beyond 16-235):

Attachment 6450

Histogram with levels adjust (slight spillover at left is usually OK):

Attachment 6451
However, my histogram looks more like this:

At (almost) default brightness:
Histogram_1_Default.jpg

At reduced brightness:
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Close up:
Histogram_2_Lowered.jpg

I am using a Tevion USB / ATI clone purchased here from Lord Smurf. It doesn't seem to matter what capture chain I use (JVC TBC VCR, JVC non-TBC VCR, through ES10 and ES15, with or without external TBC) so I'm sure it must be the USB. Pushing brightness to max just moves the 'clump' along the histogram - sometimes it spreads out a little, but if it does it comes at the cost of clipped whites.

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Is the Tevion crushing the blacks?
I'm capturing home made and dubbed tapes. Is it possible they've been 'pre-crushed'?
Should I let the white clip a little bit, for the benefit of slightly less crushed blacks?
Should I be concerned about this at all or is it normal??

Thank you


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  #2  
06-05-2021, 11:55 PM
Hushpower Hushpower is offline
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I use Brightness to move the left edge and Contrast to move the right edge. Set left then adjust right.

Sanlyn's comment re only working during Preview is incorrect (assuming he's talking about the blue stuff on the histogram); I can control it during capture as well (USB3HDCAP, GV-USB2 and 710-USB using Graphstudio).
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  #3  
06-06-2021, 05:27 AM
timtape timtape is online now
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Reduce the contrast.
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  #4  
06-06-2021, 08:01 AM
lollo2 lollo2 is offline
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Quote:
The black end of my histogram never goes into the red
Because your card cannot capture 0-16.
When you reduced brightness you shifted the whole histogram to the left. To do not loose blacks you should act on contrast, as timtape said, to shrink the histogram.

However, your card can capture 235-255 (right red zone), so I would not do anything at capture level because the first histogram looks quite good, and adjust the levels in post-processing.
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  #5  
06-06-2021, 11:32 AM
traal traal is offline
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Except the first histogram shows, or strongly suggests, clipping at 255. But reducing the contrast will help.
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  #6  
06-06-2021, 12:16 PM
lollo2 lollo2 is offline
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Difficult for me to understand a "natural" clip for Y>255. I suspect that the sharpness at 2 in the settings is adding some "sharp edge" causing that.

Eventually kingbean can try to set the sharpness at 0 and see if the first histogram changes in the right side
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  #7  
06-06-2021, 05:02 PM
kingbean kingbean is offline
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Thanks all.

Quote:
Originally Posted by lollo2 View Post
Because your card cannot capture 0-16.
When you reduced brightness you shifted the whole histogram to the left. To do not loose blacks you should act on contrast, as timtape said, to shrink the histogram.

However, your card can capture 235-255 (right red zone), so I would not do anything at capture level because the first histogram looks quite good, and adjust the levels in post-processing.
This is interesting. I had understood the histogram to be showing me the incoming signal, and anything outside of the 16-235 range would be lost after capture. But, it's actually showing what my capture USB is capable of capturing?

One other question: if the histogram shows only blue at default settings, and I have some room to spare on either side before I reach clipping - should I act to expand the blue range as muc as possible??

I will have a play around with contrast and sharpness later today and report back.
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  #8  
06-07-2021, 01:25 AM
lollo2 lollo2 is offline
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Quote:
But, it's actually showing what my capture USB is capable of capturing?
Yes.

Quote:
One other question: if the histogram shows only blue at default settings, and I have some room to spare on either side before I reach clipping - should I act to expand the blue range as muc as possible??
Your card cannot capture Y<16, so you will never enter in the "red" zone for the blacks. The procamp setting are used to capture inside the range of a card, very often being 16-252/253/254.
You should find the worst case for your video, and expand/shrink the histogram to be sure to stay inside the "capture" range. (in your case it will be "red" on the right side).
You can take some margin if you are not sure about the worst case, but in this case the "blu" on the left is not expanded as much as possible because your limit is 16 not 0, so taking some margin will force you to fix the lower limit at something like 20.
Be aware that in this case, if you plan any later restoration, the Y>235 levels must be fixed.

In alternative, you can capture 16-235 (only blu range). IMO the first is more effective, because playing too much with the procamp setting in the cards often induces "spiky / saw tooth histogram" leading to banding:
https://forum.videohelp.com/threads/376945-Conventional-analog-capture-workflow-vs-alternative-methods-comparison
https://forum.videohelp.com/threads/394878-450-VHS-Hi8-10-bit-422-with-Frame-Line-TBC-Capture-Setup#post2564905
http://www.digitalfaq.com/forum/video-capture/8924-causes-tooth-pattern.html
http://www.digitalfaq.com/forum/video-capture/9475-brighteye-75-first-2.html#post61473
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  #9  
06-07-2021, 04:11 AM
kingbean kingbean is offline
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Great, thanks for that info!

Quote:
Originally Posted by lollo2 View Post
Be aware that in this case, if you plan any later restoration, the Y>235 levels must be fixed.
What is involved in "fixing" the levels?

I've got upward of 1800 tapes to work through, each with multiple clips and sources. So my plan is to capture them all and archive the HuffYUV files. I won't have time for restorations at this point, but the option will be there in the future.

Some of the restoration results I've seen on here are amazing, so I'd like to restore a few videos here and there, time allowing, but for the most part it's not a priority. With that said, I still want to capture while retaining as many options for future restoration as possible.

Is capturing with Y>235 values, without any restoration or further editing, going to degrade playback quality?

I tried changing contrast and sharpness today, and it resulted in the histogram 'segmenting' itself.

Three pics, sharpness at 2 (default), then 1, then 0. I have not adjust contrast in these images but it had the same effect.

Histogram_5_Flatish.jpg
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Histogram_6_Teeth.jpg

What do these peaks and valleys mean in regard to my captured video? Is this similar to the sawtooh banding you've linked? I will have to go through those threads properly over the next few days when I've got time to soak it all in.


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  #10  
06-07-2021, 07:03 AM
lollo2 lollo2 is offline
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Quote:
What is involved in "fixing" the levels?
Simply a level correction with ColorYUV() or Levels()

Quote:
Is capturing with Y>235 values, without any restoration or further editing, going to degrade playback quality?
It depends on the player. For example VirtualDub will clamp Y>235 levels, but nobody use it as a player. You have to experiment for yourself, with your favourite PC player and your TV.

Quote:
I tried changing contrast and sharpness today, and it resulted in the histogram 'segmenting' itself.
Yes. However is not a big issue, any light avisynth filtering will fix it https://forum.videohelp.com/threads/...It#post2615773

Quote:
Three pics, sharpness at 2 (default), then 1, then 0. I have not adjust contrast in these images but it had the same effect.
You should use the same frame for comparison, otherwise is not clear
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