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Old 12th October 2020, 20:10   #26  |  Link
poisondeathray
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Join Date: Sep 2007
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Quote:
Originally Posted by feisty2 View Post
also regarding fp32 YCbCr, it is obvious that this format corresponds directly to E'y, E'cb and E'cr defined on page 3 of Rec601, so the range should be [0, 1] and [-0.5, 0.5] accordingly. There is no definition of limited range E'y, E'cb and E'cr.
f2 - maybe some confusion over terminology over what "range" refers to. It does not refer to absolute range. It refers to black and white point

1) "Limited range RGB" means 16-235 black to white (in 8bit). It does not mean that you cannot have RGB code value of 1. It describes the black to white range only . When you apply full range equations, with broadcast range input (legal range) YUV video Y 16-235 - You get limited range RGB 16-235. If you had a Y'CbCr overshoot of 236,128,128 the resulting RGB value would be 236,236,236 . It's still called "limited range RGB", because the black to white point is RGB 16-235. The 236 value doesn't get clipped

A large part of why we have "legal range video" in the first place is for historical broadcast reasons - 0,255 are reserved for sync , and the equipment/ bandwidth issues mentioned earlier



2)
"Full range RGB" means 0-255 black to white. This is what you're used to. Computer RGB. Y 16-235 => RGB 0-255. This is limited range equation application . Range expansion


But of you started with full range video (Y 0-255) , you would get clipping of Y <16, Y>235 values when limited range equation to RGB is applied. In this case, that same YCbCr 236,128,128 value does get clipped in the RGB conversion



If you look at BT.709 If you look under 4.6 - R,G,B,Y (not just Y) black level and nominal peak are listed as 16, 235 (and 64, 940 for 10bit) . Both full and limited range equations are valid for BT.709, but only the full range equation results in RGB 16-235 from legal range Y input . That's the default BT709 specification. Not surprisingly, r103 guidelines mirror that range for R,G,B,Y

https://i.postimg.cc/2jtZk1Nc/BT709-4-6.png



Hypothetically, you had unlimited bandwidth and if fp32 was a transmission format, and you still had restrictions (pretty stupid if you had unlimited bandwidth, that's the main reason for the existence of full vs. limited ranges in the first place. We'd just use RGB for everything) , then the concept of full and limited Y range would still be valid . But as mentioned earlier limiting in fp32 never exists in a real scenario. It's always done at the end

Last edited by poisondeathray; 12th October 2020 at 20:36.
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