You must be very new to After Effects. Type masking in the search help the field at the top right corner of AE check out community resources and help files.
At the most basic level you use the pen tool (g) with the layer selected and click and drag a path. At the most advanced level you use color channels and color controls to generate a track matte or key.
I would visit the AE product homepage and click on the learning link at the top right corner and go through several of the tutorials.
Past Rick's comment, if you just want to fill the frame with part of an image, you can scale then position the image, as in PrPro ... scale to the basic size, then position it in the frame.
Layer/Transform/Scale ... then either move it by selection tool or via Ctrl/Cmd-Shft-P to bring up the dialog box in Layer/Transform.
Thank you for answering my question. I am relatively new to AE. I have used the pen tool to create the mask, but could not figure out how to use just the masked object and have transparency in the rest of the frame. I have been using the tutorials, but I so far have not been able to find information I what I wanted to do. I think I have basic misunderstanding of how AE is working with images. Thanks to your time.
I will say it again. Type "mask"in the search help field and read up. If you just draw a mask with the pen tool it will include everything inside the mask. If you change the mode to subtract them and I will exclude everything inside the mask.
It will take you years to learn how to use this program by just poking around and experimenting. You have to study.
I have used the pen tool to create the mask, but could not figure out how to use just the masked object and have transparency in the rest of the frame. I have been using the tutorials, but I so far have not been able to find information I what I wanted to do. I think I have basic misunderstanding of how AE is working with images.
Here's the good news: AE's default condition is transparency. You don't have to do anything to get it -- if no layer is blocking the background, it's transparent. You simply have to render your comp in a video codec that supports alpha channels.
Now the challenging news: Rick's right about studying AE. You start with the basics, and work your way up. Photoshop experience counts for very little. If you've got the dough, Lynda.com has good AE training. If you don't, here are a couple of really good freebies:
These two offer structured, topic-by-topic information so you can wrap your head around the way AE works. If you try to learn a couple of tricks in AE, you'll get lost.
Thanks to you all for your help. I will continue my study with the suggestions.
Working in PrPro, one doesn't need a lot of time with alpha channels and transparency ... but from the little I know of Ae, working with alpha channels is a must-know thing.
The suggestions that you study the tut's suggested, and get probably a lynda-com subscription are well taken. One can learn a lot about PrPro by poking the box, checking the help, and asking a few questions. Ae ... um ... every option has options with options which have options.