GAMMA - What do my controls actualy do ?

[Alaric]

Hi There,

I recently picked up a BG808s and as part of the updates the S gives me is some Gamma controls.

I have two controls :

Blue Gamma Correction - Slope (0-9) & Breakpoint (0-100)
R & B Midlights (0-15)

Now i have tried standing in front of the tubes and upping these but have yet to turn invisible, get stretchy or look like brick, so i guess they are not superhero adjustments

However, having a quick play with CalMan this morning on a reasonably flat colour chart with blue arcing up ( 20&80% correctish) and red and green arcing down :

Slope and breakpoint made little difference (0->5 & 14->50). The blue tipped slightly, but red and green made more difference. My overall gamma didn't shift more than 0.04 down (2.94 to 2.90)

I then tried the R&B midlights and could see a large shift. I moved red from 3->8 and blue from 0->8. This heavily seperated the colours. Green stayed near the midpoint, Red & Blue shifted apart (can't recall which went up/down).


What i want to really know is what the controls do, what they adjust as the slope/midpoint etc mean little to me as i don't know the refference.

Reading some of Poynton's faq's they talk about Gamma being intensity and from Doom days Gamma made things brighter (shadows less dark, monsters didn't hide so well), however what i figure overall Gamma does is compress or increase the way with which the light level alters, ie increasing gamma makes the light output at 10% & 90% apear closer...However that seems basic and simplified for what controls i have ?

Any help/advice etc ?

Ta,
Lee

BTW how do i edit the title to what !

[IronBear]

Lee - Gamma is the coefficient that ties the signal level into the actual output light level. Since the gamma curve is a power curve, you have to look at it three parts (low, mid, high). At the low end, if your gamma is too large, then you will not have a sufficient increase in light level from one signal level to the next. This leads to "black crush" and lost shadow details.

At the mid-level, displays tend to be relatively less susceptible to gamma issues.

At the high end, if your gamma is too low, then you again do not have a sufficient increase in the light level to distinguish one signal level from another. This is one of the major potential causes of "white crush" (after having contrast set incorrectly). Since digital displays try to increase their On/Off CR, they tend to blow out the gamma. This is why the Samsung 710 has better shadow details despite the lower on/off CR than many of its competitors.

All that, though, is irrelevant for your Barco. Given that Gamma is a performance metric CalMAN was designed to give users a lot of information about, then you should be able to see what happens when you move your controls. Document your starting values, put in a decently-sized change to one of the controls and measure with a grayscale run. If you can post some PDFs, we should be able to see what's going on.

Later,
Bill

[Alaric]

Hi Bill,

I have had a play with the gamma controls and they seem to help me get some better colour tracking curves....However the more i play with this thing, the more i find almost everything impacts on everything else....I wish something would stay constant !

It also seems the more controls i have the harder things are to actualy achive. My data without green or gamma was relativly easy as there was little i could do about either

I've posted my current attempt...I'm still nudging values by 1 or 2 points each run and finding i am still improving overall, though sometimes i end up moving a colour a point, moing another and going back and moving the first one back as sometimes colours seem to mask one another !

My settings are listed in the notes and the wizard area as i find if i don't and i tweak something i then find it hard to go back as the Barco pj uses directional arrows for inputs which can often mean hitting one way to find the value actualy alters it

I've also inclided a second page three as changing to 20% on the RGB Tracking alters the graph scale so that a flat line at 0% actual shows a better representation of whats going on....when i first got close and the scale changed it scared the whatsits out of me !

0% is a fake value as i find it hard to imposible (levels change with colour/light output) to get a reasonable reading for this and to be honest unless black is highly tinted then its black anyway !

I'll have a go at creating some PDFs of Gamma changes. Curiously Gamma is an overall PJ setting than input related, so its set once for the whole PJm thought ATM i'm only using a HTPC.

Thanks,
Lee

[Alaric]

Hi there,

Right. I've the first two PDF's here. I haven't altered any colour settings, brightness/contrast etc.

Factory Preset - Set the gamma back to this. Documented what my PJ's settings are in the notes box. I will be using this as the base line.

Blue Breakpoint - From the Factory preset (above) i increased the blue breakpoint from 14-90

Cya,
Lee

[Alaric]

Hi There,

Next 2

Blue slope - From 6 down to 1, the minimum

Blue Midlights - 0 to 15

cya,
Lee

[Alaric]

Hi There,

Last one :

Red Midlights from 04-15

Now i know all of these files are gross adjustments and i'm sure they interact, so if someone wants me to adjust two or more of these to any particular setting, let me know, it only takes a few minutes for a run through.

I've also noticed that shifting the colours also affects the big gamma curve, so surgestions there are also welcome. My current setting (first files above) is still a work in progress (hence the note of blue gain+1 as that was my last thought on things)....The colours versus the inbuilt temperatures are a lot better, however watching Enterprise:BrokenBow last night Archers face seemed to shift from scene to scene and perhaps looked a little orange in places....i didn't notice this so much in other people. Also in places it was very dark, though i probably need to re-visit contrast/brightness (i'm never happy with these) as since setting them i'd done a lot of work on the colours which i'm sure will affect things.

BTW - When i picked this PJ up there was a red coloured Gel infront of the red tube (behind the lenses). I've left this in for the moment, as i presume Roland put it there (he installed the PJ prior to me getting it_

Cya,
Lee