RGB/YUV Pixel Conversion

Conversion back and forward between RGB and YUV formats is a topic that often causes confusion - are we dealing with gamma corrected RGB values ? Which luminance/chrominance colour space are we actually dealing with ? Why can't someone just tell me how to do the conversion without giving me 5 chapters of theory first !

To cut a long story short, here are the formulae that I have used to do the conversions for PC video applications. The colour space in question is actually YC_bC_r and not YUV, which a video purist will tell you is, in fact, the colour scheme employed in PAL TV systems and is somewhat different (NTSC TVs use YIQ which is different again). Why the PC video fraternity adopted the term YUV is a mystery but I strongly suspect that it has something to do with not having to type subscripts.

The following 2 sets of formulae are taken from information from Keith Jack's excellent book "Video Demystified" (ISBN 1-878707-09-4).

RGB to YUV Conversion

Y  =      (0.257 * R) + (0.504 * G) + (0.098 * B) + 16

Cr = V =  (0.439 * R) - (0.368 * G) - (0.071 * B) + 128

Cb = U = -(0.148 * R) - (0.291 * G) + (0.439 * B) + 128

YUV to RGB Conversion

B = 1.164(Y - 16)                   + 2.018(U - 128)

G = 1.164(Y - 16) - 0.813(V - 128) - 0.391(U - 128)

R = 1.164(Y - 16) + 1.596(V - 128)

In both these cases, you have to clamp the output values to keep them in the [0-255] range. Rumour has it that the valid range is actually a subset of [0-255] (I've seen an RGB range of [16-235] mentioned) but clamping the values into [0-255] seems to produce acceptable results to me.

Further Information

Julien (surname unknown) suggests that there are problems with the above formulae and suggests the following instead:

Y = 0.299R + 0.587G + 0.114B

U'= (B-Y)*0.565

V'= (R-Y)*0.713

with reciprocal versions:

R = Y + 1.403V'

G = Y - 0.344U' - 0.714V'

B = Y + 1.770U'

Scott Scriven has recently sent me a C program he wrote to convert Quantel YUV files as generated by SGI's "ZapIt!" video capture software into RGB PPM files. You can get hold of the source by clicking here. Please contact Scott with any support questions on this program.

Intel actually has an extensive web site on the topic of colour space conversion and you can find it here.