If I understand your request, you want scrolling credits that fade at the top and bottom of the screen and this to be on a video background ? This can all be done in after effects or you can import your scrolling credits on black background as you have suggested you may already have, I will discuss both.
EG1 – Credits text generated in After Effects
1. Create a comp with your background video.
2. Add a text box by clicking the text tool and dragging to create a bounding box.
3. Enter your credits into the text box and resize the bounding box if necessary.
4. Create the text scroll animation by going to the start of your comp, position the text layer below the bottom of the comp, click the stopwatch for the position then go to the end of the comp and move the text layer up above the comp..
5. Add a solid white layer.
6. Add a rectangular mask to the solid by selecting the rectangle Q and then double click it.
7. Adjust the top and bottom of the mask so that there is a small area below the top and above the bottom of the comp (so vertically it sits within the comp). Also, expand the mask past the left and right edges of the comp.
8. Feather the mask and adjust the expansion negatively to pull it in. This creates the gradient top and bottom. As the feather / expansion applies to the whole mask, this is why we made the left right edges bigger than the comp in Step 7 above.
9. On the text layer, select Alpha Track matte. The white solid with the mask on now acts as the gradient mask.
EG2 – Using externally rendered scrolling text on black background.
1. Open comp with background video
2. Drag in the scrolling text video layer
3. Create a white solid and follow step 7 & 8 & 9 from EG1 above
4. On the text layer. change the transfer mode from normal to Add.
I have uploadad a sample project for reference. Hope this helps
7547_fadingcredits.aep.zip
Declan Smith
https://www.madpanic.tv
After Effects CS6/ FCS3 / Canon XLH1 / Canon 7D / Reason / Cubase
“it’s either binary or it’s not”