Sky replacement using blue-channel

[DJMartin]

Hello, I am trying to a sky replacement by extracting the blue channel. I have done some replacement before, but not by using the blue channel. And now I am kinda stuck, I don't got a clue what I shold do.

This is how far I have come so long:


I've took a live footage (Image 1).
Duplicated it and then allplied levels to raise the blue contrast. (Image 2)
In the comp window I selcted the bluechannel, so only that will show (Image 3)
My question: How can I use the luma matte in image 3, so it will apply to image 1? If I just set it as a trackmatte it does not work, since the "view blue only" is just for preview purpose and affect the whole comp, not only the duplicated image.

Thanks for any suggestions, and sorry for my bad english, I've hope you understand:)

[bimdas]

those channel buttons are only for previewing purposes. To use the blue channel of an image as a trackmatte, use 'shift channels' effect to set all color channels to blue.

[DJMartin]

Thank you for your answer, but after trying to use the "shift channel" effect on a still image, I still not get it to work.

Here you can see what happend:

Original image
With Shift Channels set to blue (all exept alpha)
Adjusted with levels
Used as a track matte on original image with a backgroundAs you can see, it works as a greyscale function, the cameras dissapear becuase they are whiter than the sky, I am not using the blue channel as a matte, just the luminosity. If you know what I mean...

[bimdas]

I don't quite get what you are trying to do.
If you just want to get rid of the sky, a simple key would do.
Creating luma mattes from colour channels is usually used in conjunction with roto and garbage masks to separate foreground objects. You'd have to mask add those cameras in since there is little contrast between them and the sky.

[anthony113]

just use keylight, it'll get the job done fast!

[DJMartin]

Yes, I know that keylight will work very well in this case, but I was trying other methods too. But it did not work out. Thank you for your help:)

[hype]

yes, when using the blue channel, you will DEFINITELY have to roto back some areas in those examples you've posted. bimdas is correct. This goes for all channels, not just the blue.