On Sunday 13 April 2008 06:17, Ichthyostega wrote: > Hi! > > this is a often reported problem, while the circumstances under which it > arises are somewhat complicated and difficult to track down. (I was suffering > from it myself at times). Let me explain the situation a bit... > > When you use the X-xv, you use a different method of outputting video images, > which is usually hardware accelerated. This XV-output is showed as an overlay > over the normal output produced by your X server. Now, under some > circumstances it can happen that both output methods produce a different > visual result from the same pixel data. A common situation is when some gamma > correction is involved. For example, the X server may be configured to > correct for your monitor's gamma, i.e. it modifies the pixel values on the > fly. In most cases this means making dark regions brighter (as monitors often > have a too steep gamma curve, so that even dark grey pixels (e.g. 15% grey) > are already displayed as black. And if the xv output method doesn't apply the > same gamma correction (for example it doesn't apply any correction at all), > then the visible colours are different. > > Now comes the part specific to cinelerra: this application reads input data > from your source media, decodes it, probably manipulates it and then sends it > to output again. But there is a special facility which detects if the image > data isn't modified at all, in which case it could use a faster method > employing the X-xv display. But the moment the application has to do some > modifications to the video frames (e.g. when you move the fader away from > 100% or if you use the compositing facilities of cinelerra (the "camera" or > "projector") to offset the frame, then it does this manipulations using the > CPU and outputs the resulting data differently. The switch-over may even > cause a short flicker on some graphic boards. > > The problem now is: the judgement what is the "right" display depends on your > specific circumstances. If you work with image data and do a > colour/brightness correction, than this includes inadvertently the display > characteristics of your monitor. If you look at your results on another > monitor or using another gamma correction, suddenly everything seems to be > wrong. That's the reason why professional setups use these expensive colour > calibration and correction profiles. > > hope this helped clarify the situation a bit > Greetings > Hermann Vosseler
Thanks, that explains it. But there is one thing. You said that the xserver may correct gamma. In my case, this is not the case (I used nvidia-settings to set it to 1.0). My monitor (Eizo CRT T766) also displays black as pure black, and the 15-15-15 RGB was clearly visible as grey, so there is no need. Anyway, can Cinellera also do any gamma correction? You see, the thing is, when the composite is playing, the brightness is good. But, when I pause it, it becomes too bright. So, the overlay seems to be correct, but the still image Cinelerra puts up seems to be gamma corrected. I had also experienced the flickering BTW. The X11 output driver may not scale very well, but it does give the best preview. Except that with this driver, the output is always too bright. And so is the render (however, the encoded avi from the rendered frames is good again, which I don't understand; see other post in this thread). Greets. _______________________________________________ Cinelerra mailing list [email protected] https://init.linpro.no/mailman/skolelinux.no/listinfo/cinelerra
