Why we use a complex equation when do convert RGB to YCBCR instead of a simple one?

Question

When I do implement RGB2YCBCR on verilog. I saw several ways to convert RGB to YCBCR. Some of it are very simple. But others are quite complex.

For example, if we use

y = (r >> 2) + (g >> 1) + (b >> 2);

it is very easy to implement to hardware

But some people say that

y = (r*4899 + g*9617+ b*1868+ 8192) >> 14

is faster???

I don't know why they have this equation and why a complex equation better than a simple equation. And WHY THEY USE IT instead of that simple equation.

P/S: I saw that OpenCV also uses that complex one. Thank a lot!

hi there, I read this article, but I didn't find the answer to my question — Luan Nguyen, Dec 07 '17 at 17:03
_simpler_ doesn't mean _correct_. Those constants aren't there to make it more difficult, but to make it right according to the standard — Miki, Dec 07 '17 at 17:10

score 1 · Accepted Answer · 2017-12-07T23:49:35.547

1

On processors with a multiplier and a barrel shifter, all these operations take a single clock cycle and the second formula cannot be faster (5 ops vs. 7).

The complex equation is mandated by a standard coming from the television technology, with historical and technological reasons (https://en.wikipedia.org/wiki/Rec._601).

Color spaces is a rather complex topic.

Note that the first formula is not normalized: if the input values are in range [0,1], the outputs are in [0, 10], and you should divide by 10. This would make the first formula much slower.

edited Dec 07 '17 at 23:49

answered Dec 07 '17 at 19:26

Can you tell my why we use y = (4899R + 9617G + 1868B+ 8192) >> 14 instead of Y = 0.299R + 0.587G + 0.114B. I think it's the same equation. – Luan Nguyen Dec 09 '17 at 14:46
Obviously to stay in integer arithmetic. – Dec 09 '17 at 15:55
Any idea if I do equation y = (153R + 300G + 74B + 512) >> 9. – Luan Nguyen Dec 09 '17 at 16:46
@LuanNguyen: this is less accurate. – Dec 09 '17 at 17:24
why y = (4899R + 9617G + 1868B+ 8192) >> 14 is correct? why y = (153R + 300G + 74B + 512) >> 9 isnt correct?. I think they have same result – Luan Nguyen Dec 10 '17 at 17:23
@LuanNguyen: no they don't. – Dec 10 '17 at 17:48

Why we use a complex equation when do convert RGB to YCBCR instead of a simple one?

1 Answers1