Gamma Shift in Output!

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Gamma Shift in Output!

Postby J.Wajer » Thu Jun 11, 2009 18:55

Images appear considerably darker in VCube than the original. Here is an example:

Image
Original

Image
Playout from VCube

Since VCube is only a player, there is nothing I can do to correct this Gamma shift. My settings are in order.
Now I recall someone telling my about two quicktime formats with different gamma rates. Is it possible that VCube uses the wrong format? Moreover, one of my customers complaint about this shift.....and I need an reliable answer :)
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Re: Gamma Shift in Output!

Postby J.Wajer » Tue Jun 23, 2009 13:58

After further investigation it seems that the current Quicktime PC-distribution (post 7.4.5) causes this gamma-shift....but strangely enough, there is no notice of this phenomenon anywhere on the net :)
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Re: Gamma Shift in Output!

Postby J.Wajer » Wed Jul 01, 2009 17:53

nobody else witnesses this phenomenon?

ai triple poster :)
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Re: Gamma Shift in Output!

Postby alphie » Fri Oct 09, 2009 16:01

Which output of VCube are you referring to when you talk about gamma shifting?
The preview window of VCube displayed from the graphics board of the PC?
The analog output of an AJA video board (either in component or composite)?
The SDI output of an AJA video board as seen through external conversion (like an SDI compatible video monitor or the analog output of another video deck like a Digi Betacam in input to a video monitor)?

What is the "source" you are comparing VCube to (QuickTime Player as seen on the same PC screen or a tape deck with the original video displayed to a monitor)?

I ask this not because I have a software patch for VCube in mind but because source and duplicate video is more or less degraded depending on the copy methods and formats used. One factor that plays an unpredictable role when comparing the results of video copies is not the video itself but the tolerances of the display device and its integral components which may compensate for tachnological shortcomings.

Gamma correction is a term which refers to non-linear luminance encoding and decoding present in many forms, whether hardware based or software based, that may alter the perceptual values of displayed picture. VCube is far from being the only link in your display chain that potentially alters the gamma of what you may be seeing as a defect in the luminosity of a picture. In addition to the gamma slider in VCube which allows for the adjustment of VCube's gamma to your PC's graphics board QuickTime interprets gamma, your graphics card itself, your PC monitor, your video monitor, the convertors of your AJA board, another video deck with convertors, etc.

Whatever discomfort this slight shift in gamma, perceived by you and your client, may cause while comparing VCube and some other "source" may produce it will only continue to vary from display device to display device wherever you look. Neither VCube nor QuickTime have cornered the market on gamma correction. Think of a large Hi-fi store that lines up dozens of TVs for display all showing the same source. Have you ever noticed how they all seem to display picture differently? They all have the same source and yet they look different. None of them have altered the source signal but simply try to reproduce it perceptually as best as possible. Every manufacturer includes some simple adjustments in their menus to make the sets more comfortable in the environment of the individual customers’ homes but these are corrective perceptual adjustments, not calibration adjustments. VCube allows for adjustment of the gamma to the preview window (thus the graphics board of your PC) but does not control how the stand alone QuickTime player will display (nor should it) to your PC's monitor. This logic follows though the entire chain of devices to any display from any digital image media source.

This phenomenon is a long standing industrial reality and every manufacturer may use their own power law interpretation tables to fit their own technology. We don't control them nor do they control us so we choose to allow for user corrections and hope that our clients are at least as satisfied with this they may be with their video display gear. :-)
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