Green screen and cropping query

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Nigel Scott

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Green screen and cropping query

PostThu Jan 23, 2020 4:48 pm

Hi All
I have used my big TV to create a green screen background for my presenter. I made a green .jpg and loaded into the TV. I wanted my presenter to be minimised at the bottom left of the screen superimposed on the clips he was talking about. I used the color page to key the green screen out. This worked very well but when I cropped out all the superfluous stuff round the TV I found that I could not get the background clips to show. I don’t understand why.
I got round it by delivering the complete green screen clip and adding it to a new project. I could then resize, crop, and position it as I wanted. Although this works there are 2 stages and more work which is a pain as I have quite a few to do. Is there an easier way?

Regards
Nigel
Windows 10 64-bit
Intel Core i5-4690K CPU @ 3.5 GHz, 32.0GB RAM, NIVIDIA GeForce GTX 1050 Ti
Davinci Resolve Version 16.1.2
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David_Cox

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Re: Green screen and cropping query

PostThu Jan 23, 2020 7:25 pm

Did you add an alpha output on the color page? This is the bit that transfers your color selection in the color page to a transparency that the edit page will respect and show background clips underneath.
DC
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Nigel Scott

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Re: Green screen and cropping query

PostThu Jan 23, 2020 9:32 pm

David_Cox wrote:Did you add an alpha output on the color page? This is the bit that transfers your color selection in the color page to a transparency that the edit page will respect and show background clips underneath.
DC


Yes I did that. The cropped part had transparency but the rest of the image was black and this hid the clip that I was using as a background.
Windows 10 64-bit
Intel Core i5-4690K CPU @ 3.5 GHz, 32.0GB RAM, NIVIDIA GeForce GTX 1050 Ti
Davinci Resolve Version 16.1.2
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David_Cox

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Re: Green screen and cropping query

PostFri Jan 24, 2020 12:11 am

My guess is that this is to with having to invert the key. The color page is designed to isolate colors to work on. By selecting a given color, the resulting alpha channel represents the area filled by that color. But with a chroma key, you want the opposite - for the alpha channel to exclude the selected colors. So the key needs to be inverted and my guess is that this is inverting the normally transparent area outside the frame, making it opaque. This gets revealed when you shrink the frame down and it masks your background.

A quick fix is probably to keep your keyed clip full size, then it into a compound clip and resize that instead. A compound clip is a container that has it's own transparency and resize ability, so should act as you expect.

DC
Showreels: vimeo.com/davidcoxtv
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Nigel Scott

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Re: Green screen and cropping query

PostFri Jan 24, 2020 10:43 am

David_Cox wrote:My guess is that this is to with having to invert the key. The color page is designed to isolate colors to work on. By selecting a given color, the resulting alpha channel represents the area filled by that color. But with a chroma key, you want the opposite - for the alpha channel to exclude the selected colors. So the key needs to be inverted and my guess is that this is inverting the normally transparent area outside the frame, making it opaque. This gets revealed when you shrink the frame down and it masks your background.

A quick fix is probably to keep your keyed clip full size, then it into a compound clip and resize that instead. A compound clip is a container that has it's own transparency and resize ability, so should act as you expect.

DC


Thanks David. That makes sense. I will try that now and let you know if your guess is correct.
Windows 10 64-bit
Intel Core i5-4690K CPU @ 3.5 GHz, 32.0GB RAM, NIVIDIA GeForce GTX 1050 Ti
Davinci Resolve Version 16.1.2
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Nigel Scott

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Re: Green screen and cropping query

PostFri Jan 24, 2020 11:33 am

Nigel Scott wrote:
David_Cox wrote:My guess is that this is to with having to invert the key. The color page is designed to isolate colors to work on. By selecting a given color, the resulting alpha channel represents the area filled by that color. But with a chroma key, you want the opposite - for the alpha channel to exclude the selected colors. So the key needs to be inverted and my guess is that this is inverting the normally transparent area outside the frame, making it opaque. This gets revealed when you shrink the frame down and it masks your background.

A quick fix is probably to keep your keyed clip full size, then it into a compound clip and resize that instead. A compound clip is a container that has it's own transparency and resize ability, so should act as you expect.

DC


Thanks David. That makes sense. I will try that now and let you know if your guess is correct.


OK David. That worked perfectly! Many thanks you have saved me hours!

Nigel
Windows 10 64-bit
Intel Core i5-4690K CPU @ 3.5 GHz, 32.0GB RAM, NIVIDIA GeForce GTX 1050 Ti
Davinci Resolve Version 16.1.2

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