[mythtv-users] Colour Saturation Problem

Ian Trider iantri at gmail.com
Wed Jun 1 13:52:53 UTC 2005

On 5/31/05, Brad Fuller <bradallenfuller at yahoo.com> wrote:
> good idea.
> But, doesn't this just test the capture ability of the card?
> Furthermore, I would expect that a capture via composite and then via RF
> would show differences.

Essentially correct -- that is why (sorry, I don't think I explained
this well), you use an image editing program to check that the card is
capturing the image correctly -- i.e. if the test pattern is supposed
to have a 75% saturated red, green, and blue bar, when you load it up
in the GIMP and use the eyedropper to select the colour, it should
give you 75% saturation, with a hue (er, I think -- sorry, don't know
gimp) that is fully red.

Of course, you won't get this bangon the first time, which is why you
tweak the capture settings a bit to try and get it closer to accurate,
then capture the test image again and check the colours in GIMP.  Once
you know it is capturing correctly, you can output a captured test
pattern via your video card and adjust the video card's colour
controls until the image displays on the TV the way it is supposed to.
(Your DVD should have come with coloured filters you view it through
through to adjust the hue -- except, you've already adjusted the TV to
the DVD player, so you adjust the video card hue controls until the
image that comes out on the TV is correct)

To clarify on what I've said above:

> Wouldn't you also need to calibrate the output of the card with images
> that are known to be correct?

> If true, how does one calibrate the input and also the output of video
> cards?

Yes.  Capturing the DVD test pattern, and checking the values in GIMP
will tell you whether it is capturing correctly -- once you have the
capture settings accurate, you have a known good image that you can
output to the video card.

There might be differences in the RF input.  Of course, there might
(will) be differences between between different composite sources. 
And to make it worse, there will be difference between channels on RF

But, if you'd like to calibrate the RF input, after you've gotten
everything else done, find a channel that runs a test pattern during
the night (many local network affiliates will), and capture a test
clip -- check the values in GIMP, adjust, rinse, repeat.

No, it's not a lot of fun. :)  And yes, my description was extremely
confusing. Sorry. :(

Ian Trider
iantri at gmail.com

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