Is YUV that lossy on RGB displays ? computer, displays, captures, contrast

andy

Why are the quot;purequot; colors of the THX calibration picture on THX DVDs so dark ?

I've captured the THX calibration screen in the Tron DVD, and pasted true RGB colors with Corel Photopaint next the DVD ones.

THX colors (145 kB)

I also pasted VOB captures made with DVD2AVI in PC and TV scale. Now I see why I must turn the contrast of my computer screen to 100% to watch movies, and turn it back to 40% to work with Windows.

Why are DVD so dark ? Is it a inherent loss when transcoding from YUV to RGB ?

Welcome, Pio2001, didn't saw you here before, lurking huh?

gt;EDIT : sorry, I see it's a chat section... could someone move the thread to the appropriate forum, if there is one ?

Moved to DVD2AVI and banned you, BTW  .

Read this and this.

It seems neither of those links talks about gamuts inconsistencies between YUV and RGB.

I used the formulae given is the first link to check RGB values :

RGB to YUV

Given:

Kr = 0.299
Kb = 0.114
L = Kr * R + Kb * B + (1 – Kr – Kb) * G

Then:

Y = round(219 *  L / 255) + 16
U = round(224 * 0.5 * (B - L) / ((1 - Kb) * 255)) + 128
V = round(224 * 0.5 * (R - L) / ((1 - Kr) * 255)) + 128

YUV to RGB

given:

C = Y - 16
D = U - 128
E = V - 128

the formulas to convert YUV to RGB can be derived as follows:

R = clip( round( 1.164383 * C                   + 1.596027 * E  ) )
G = clip( round( 1.164383 * C - (0.391762 * D) - (0.812968 * E) ) )
B = clip( round( 1.164383 * C +  2.017232 * D                   ) )

where clip() denotes clipping to a range of [0..255].

I thus calculated the YUV values of the basic colors :

Black = 16, 128, 128
red = 81, 90, 240
green = 145, 54, 34
blue = 41, 240, 110
cyan = 170, 166, 16
magenta = 106, 202, 222
yellow = 210, 16, 146
white =235, 128, 128

So these value should be the ones used in the DVD.

Then I calculated the RGB rendering of them :

black = 0, 0, 0
red = 254, 0, 0
green = 0, 255, 1
blue = 0, 0, 255
cyan = 1, 255, 255
magenta = 255, 0, 254
yellow = 255, 255, 0
white = 255, 255, 255

The RGB-gt;YUV-gt;RGB conversion is quite perfect.
I compared to the values got in the DVD captures (DVD2AVI vob, PC scale, captured in Corel Photopaint in a 5x5 pixels sample) :

black = 0, 0, 0
red = 188, 0, 7
green = 7, 189, 7
blue = 7, 0, 188
cyan = 7, 190, 187
magenta = 188, 0, 189
yellow = 190, 191, 7
white = 254, 254, 254

Only the white and black are correct. Other colors are completely darkened.

Then I calculated what happens using the 8 bit precision formulae. I only did the yellow calculation :

YUV = 211, 16, 146
RGB = 255, 255, 2

The result is again nearly perfect. The darkness I got doesn't come from RGB/YUV conversions, or shouldn't.

So the question remains exactly the same : why are DVDs so dark ? Can anyone with a calibration chart (this one comes from the R1 Tron 20th anniversary collector's edition DVD) make a Vob capture of it (or a WinDVD 2.x capture of it, as its the only DVD soft we found that makes raw captures)?

It could have something to do with the video card. Many video cards (like nVidia) have options to adjust the brightness of the overlay, this would affect brightness of playback...

It seems unlikely since I can't capture anything from the overlay. The pictures are directly converted from the Vob, or captured by WinDVD.
Also, in France in the 80s-90s, there was such a calibration broadcasted after the last programs of the evening, on the national SECAM airy channel, and at this time without computer, DVD, nor even VCR, I already noted that the yellow was not yellow at all. It must be a general characteristic of video playback (brighness calibration), or the colors used in calibration patterns are not RGB (I can't imagine they are CMYK based, but I'll check the RGB values of CMYK, just in case).

Hello again !

I think I've found my answer. I recently got the Animatrix DVD French edition (PAL R2), and captured some pictures of the quot;second renaissancequot; anime, where the colors are very bright.

Red RGB value reaches (255, 21, 0), while the THX red is (188, 0, 7)
Yellow can be (255, 241, 0) while THX yellow is (190, 191, 7)
Cyan reaches (0, 230, 249) vs (7, 190, 187).Is there some kind of NTSC calibration that comes into play, or is the THX chart just useless ?

.

There are two kind of color bars used in video. 100 % color bars, and 75 % color bars. These ones are 75 %, and thus have theoretical 8 bits RGB values of 191. Mine are 100 %, with 255 values, and thus are brighter.

Thanks anyway for those who tried to help me to understand.