Trent Piepho (xyzzy@xxxxxxxxxxxxx): > > I have a bttv-compatible card (BT878) and I was noticing that my > > images are too 'bright'. I quickly discovered this is because the > > BT878 samples the luma channel in the range 16-253 instead of the > > normal 16-235 specified by Rec. 601. > > I don't think you're correct about this. The bt848 DOES use the CCIR > 601 colorspace with a luma excursion of 16-235. If you take the yuv > output of the bt848 and convert it to RGB with the same equations you > would use for rec 601 input, you will get the correct output. [...] Yeah, on Friday night I found a station broadcasting some colour bars which let me look more closely at the values. Ugh. I'm still unconvinced about what to do with invalid values. Just passing them into the 601 conversion seems to be the equivalent of clamping. I've also found the chroma to often have an excursion of about 3-253. If I scale luma to 16-235 (from 16-253) and chroma to 16-240 (from 3-253, centered at 128), then I _seem_ to get more definition from my colours. Thoughts? I should probably capture in RAW mode and do the math myself to compare. :( One problem I had was that I was assuming my video card's conversion wasn't changing the values before displaying them. As it turns out, the default 'Xv brightness/contrast' settings for my driver weren't normalized. -Billy -- Billy Biggs vektor@xxxxxxxxxxxx
