Alpha channel trouble in Motion

[Doyle Rockwell]

Heya,

It sounds like the flag footage doesn’t have an integrated alpha channel (not an RGBA image), but that they gave you a second clip to use as a masking source. The second clip is probably an RGB black-and-white image, so you will need to use the luminance of it as your mask, not the alpha. You can accomplish this by using an Image Mask. Here’s what you do:

1. Add both clips to the Canvas, starting on the same frame. Aligning them positionally gets you points for neatness, but isn’t necessary. We’ll call the clips “Flag” and “FlagMatte”.

2. Select the Flag clip and go to Object->Add Image Mask (or press Cmd+Shft+M). This adds an image mask (doesn’t have a source, yet) to the Flag clip.

3. Select the image mask (in the Layers List) and view the Image Mask tab of the Inspector.

4. Drag-and-drop the FlagMatte clip into the Mask Source image well. This will automatically disable (turn off the visibility of) the FlagMatte clip, as you no longer need to see it.

5. Disable the ‘Stencil’ checkbox, as you want the mask to be centered so that it is properly registered.

6. Set the Source Channel popup menu to ‘Luminance’.

And that should do it. The image mask applied to the Flag clip will use the luminance (brightness) of the FlagMatte clip as the source for the mask, giving you the alpha result that you’re after.

Hope this works for you. Good luck with your project!

[Holly Middleton]

Thanks for the reply. This did knock the black background out, but left the foreground as the solid
white flag rather than the checkered version. I double-checked to make sure I applied the image
mask to the flag clip and the source for the image mask was the flag matte. I had the flag matte
as the upper layer – although changing this didn’t affect the outcome. Makes me think I’m going
crazy! Any other advice?

[Doyle Rockwell]

Heya,

This is a perfect example of the kind of thing where if you’re in the same room it would be an immediate “Oh, you need to…” With text as the means of helping, the little things are the hardest 🙂

Layer order doesn’t matter when using image masks (not like the track mattes in AE). If the matte worked but you’re seeing a solid white flag, try checking the following:

– Did you accidentally set the Canvas to display the Alpha channel? This would show you a solid white flag when the masking is done correctly. Press Shift+C to return to viewing Color.

– Making the FlagMatte clip the mask source should have automatically disabled the clip. Is it still “on” (visible) for some reason? Take a look at the checkbox next to the clip’s name in the Layer’s List: the checkbox should be unchecked (clip not visible).

[Holly Middleton]

You’re exactly right about troubleshooting these things via text! I am set to view color on the canvas and yes,
the layer matte is hidden – not checked, which it did automatically when I set the source of the matte. All
blend modes are set to normal except the image mask itself, which is “add”. Thanks for the additional input…if
I find out what the missing piece is, I’ll post it … perhaps it will save someone else the frustration!

Holly

[Holly Middleton]

OK FYI…in case you’re curious…I got it to work…
When I downloaded the clip, both the flag portion and the following alpha version were one long clip.
I went back and exported them as separate clips from FCP and then it worked…so I guess it was
actually usiing the wrong portion of the clip on my first attempts. I could probably have fixed this
in motion telling it what frame to start the image mask on…but the export also fixed the problem.

Lesson learned…thanks for the help

Holly