[Interest] QVideoFrame and YUV question

[Jason H]

And from http://en.wikipedia.org/wiki/NTSC (hit send accidentally)


"In January 1950, the Committee was reconstituted to standardize color television. In December 1953, it unanimously approved what is now called the NTSC color television standard (later defined as RS-170a). The "compatible color" standard retained full backward compatibility with existing black-and-white television sets. Color information was added to the black-and-white image by adding a color subcarrier of 4.5 × 455/572 = 315/88 MHz (approximately 3.58 MHz) to the video signal. To reduce the visibility of interference between the chrominance signal and FM sound carrier required a slight reduction of the frame rate from 30 frames per second to 30/1.001 (approximately 29.97) frames per second, and changing the line frequency from 15,750 Hz to 15,750/1.001 Hz (approximately 15,734.26 Hz)."


From http://en.wikipedia.org/wiki/Subcarrier

"Likewise, analog TV signals were transmitted with the black and white luminance part as the main signal, and the color chrominance as the subcarriers. A black and white TV simply ignores the extra information, as it has no decoder for it. To reduce the bandwidth of the color subcarriers, the sampling rate for color information is reduced four-to-one by using half vertical resolution on every other scan line. (This is made possible by the fact that the human eye sees much more detail in contrastthan in color.) In addition, only blue and red are transmitted, with green being determined by subtracting the other two from the luminance and taking the remainder. (See: YIQ, YCbCr, YPbPr) Various broadcast television systems use different subcarrier frequencies, in addition to differences in encoding."



________________________________
 From: Thiago Macieira <thiago.macieira at intel.com>
To: interest at qt-project.org 
Sent: Friday, March 14, 2014 12:22 PM
Subject: Re: [Interest] QVideoFrame and YUV question
 

Em sex 14 mar 2014, às 08:47:15, Jason H escreveu:
> The frame is not laid out like RGB. This is for legacy reasons. Black and
> White TV presented a black and white frame (Y) when color TV was added, it
> was added in a backwards-compatible way

This has nothing to do with black & white TV. Digital image formats were 
invented way after colour TV.

The reason why Y is sampled per pixel whereas U and V aren't is because the 
eye isn't as sensitive to colour as it is to luminosity. By using one U and 
one V for 4 pixels, we reduce the amount of data from 12 values to 6 in each 
2x2 block -- that is, half the amount of data.

There are some other YUV formats that lay out data differently, including all 
the chrominance values, like YUV 4:4:4.
-- 
Thiago Macieira - thiago.macieira (AT) intel.com
  Software Architect - Intel Open Source Technology Center


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