Pat wrote:
Normally you can set legalisers up in two ways.
One way is a hard clip. Basically you choose the threshold and anything over that is hard clipped to that level. That could be 100% RGB and that would ensure nothing over 700mv (in the analogue domain) was allowed past. The EBU spec actually allows 103% so it's a bit more tolerant than a 100% clip.
However it's more complex than that. If the R channel for example is over but B & G are under you can't just clip R as it will affect the hue at that point. So they have to reduce the B & G signals by the same proportion even though they were actually legal.
Hard clipping is the best option for content that has been graded and adjusted to be within spec and you are just making sure nothing has slipped past. You therefore don't expect the output image to look any different to the timeline.
Soft clipping and Knees is a different option. You basically select an upper range that you will be adding compression to. So as signals approach the limit compression will start to be applied (again R G B balance being balanced) So 100% colour bars will likely be slightly compressed at the top end. Typically with legalisers set like this the bars will have to be laid out first with the legaliser on bypass and then the rest with the legaliser on. This mode works best for poorly graded content with content overshooting as the peeks are compressed rather than clipped (unless they are massive then they get clipped)
In this mode the output picture can be visibly different and if we are onlining with content that is that varied and we have the legaliser set to that mode we will show and monitor the output so the client can see what the picture will look like on tape.
I reply:
One way is a hard clip. Basically you choose the threshold and anything over that is hard clipped to that level. That could be 100% RGB and that would ensure nothing over 700mv (in the analogue domain) was allowed past. The EBU spec actually allows 103% so it's a bit more tolerant than a 100% clip.
However it's more complex than that. If the R channel for example is over but B & G are under you can't just clip R as it will affect the hue at that point. So they have to reduce the B & G signals by the same proportion even though they were actually legal.
Hard clipping is the best option for content that has been graded and adjusted to be within spec and you are just making sure nothing has slipped past. You therefore don't expect the output image to look any different to the timeline.
Soft clipping and Knees is a different option. You basically select an upper range that you will be adding compression to. So as signals approach the limit compression will start to be applied (again R G B balance being balanced) So 100% colour bars will likely be slightly compressed at the top end. Typically with legalisers set like this the bars will have to be laid out first with the legaliser on bypass and then the rest with the legaliser on. This mode works best for poorly graded content with content overshooting as the peeks are compressed rather than clipped (unless they are massive then they get clipped)
In this mode the output picture can be visibly different and if we are onlining with content that is that varied and we have the legaliser set to that mode we will show and monitor the output so the client can see what the picture will look like on tape.
I reply:
Excellent response Pat. Some wise person wrote a couple of years ago in response to a similar question on a different forum that the "art" of colour grading is to make the picture look the way you want it to, but use the tools at your disposal to ensure that very little of the picture is illegal. That way the legaliser won't change your picture substantially or visibly.
So, in FCP7 I apply a "desaturate highlights and shadows" filter as the last filter in the stack, and that gives me white highlights and black (unsaturated) shadows, so that the legaliser has little to do and won't change the picture visibly. (Haven't graded on Symphony for many years, so a bit rusty on the equivalent process, but need to get up to speed very quickly!)
Here is the EBU document that specifies legal colours: https://tech.ebu.ch/docs/r/r103.pdf
If the original poster is in the US or Japan, I'm sure there is some similar standard for those areas, if this one doesn't apply.
Alan
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