Hi Con

where can i download the original grabs? Can't seem to get any detail back from the highlights the same way i've been able to do with footage i've shot.
 
PaPa said:
where can i download the original grabs? Can't seem to get any detail back from the highlights the same way i've been able to do with footage i've shot.
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"Recovering highlights" is different on footage than screenshots

The reason is those values have already been clipped in the RGB colorspace conversion with a png screenshot from VLC

When you take a RGB screenshot, you usually use BT.709 or BT.601 for the conversion matrix , thus those values Y' 235-255 become RGB 255,255,255 (i.e. clipped)

In real footage, you are still in Y'CbCr space, and any values Y' >235 (i.e. "superbrights") can still be salvaged if your application does the correction in Y'CbCr space, or before the RGB conversion , thus you can recover the highlights (i.e. you can bring Y' values down to 235, before you convert to RGB, saving that data, and it's been confirmed on other threads that AF100 does record usable data in Y' 235-255 range)
 
Mark Smith said:
how is the conversion matrix specified by VLC?
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Most PC software players won't allow you control over the change, but have auto detection for the size of the video

Most use if width > 1024 => use BT.709
if width < 1024 => use BT. 601

1024 being the cutoff width for the square pixel equivalent of 16:9 SD PAL Widescreen (ie. 16:9 720x576 displays as 1024x576 on a PC)

By convention, HD matrix is usually BT.709, SD matrix is usually BT.601

According to my tests , VLC displays using BT.601 for both (so incorrectly for HD)

Eitherway your Y' values 235-255 are clipped by either conversion. You can only save them before conversion to RGB

Some editors don't follow convention (notably Sony vegas in 8-bit mode will use studio RGB, so highlights won't be clipped - black is "mapped" to RGB 16,16,16 (instead of 0,0,0), white is "mapped" to RGB 235,235,235 (instead of 255,255,255) , so you have "head and tail" room on both sides - although very few cameras record usuable data on the black side)
 
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So then VLC might not be the best way to grab stills from MTS streams because it uses BT601. I went with it because it appeared to be the most direct route between the original file and a grabbed still, but perhaps that is not the case.
 
Mark Smith said:
So then VLC might not be the best way to grab stills from MTS streams because it uses BT601. I went with it because it appeared to be the most direct route between the original file and a grabbed still, but perhaps that is not the case.
Click to expand...

There is not a huge difference between BT601/709 . The colors will be shifted slightly, but the luminance interpretation will be the same. 99% of regular viewers won't even see the difference. The point I was trying to make earlier is was with "highlight recovery"- and both will clip. It's too late once it's been converted to RGB with either matrix
 
I could put up the MTS clips which are all very short some place if folks wanted to see those instead of the frame grabs.....
 
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