Filming a device like a mobile or tablet on camera and getting a sharp, clear image can be a tricky thing to get right, that’s why we use something called ‘screen replacement’ which falls under the VFX (visual effects) category and can save the director and editor (also any other members of the creative team) a lot of time and money.
The process is fairly simple for the editor to add the desired footage to the screen, below is a step by step guide of how a basic screen replacement would be done;
Assuming that the camera is set to the correct settings (exposure, focus, etc) the device would be filmed with a chroma green or blue screen (green is the best for this as it has the highest contrast to the edge of the device: in most cases)
When ready, the editor will take the video file, correct any colours to match the look of the film and proceed to place motion trackers on the corners of the device so that when the device moves, the markers will too.
Once all of the points line up to the edges of the device the editor will then proceed to make sure the screen still looks natural round the edges and also create a mask for anything that may appear in front of the screen
When the mask is complete the desired per-recorded video (or still images) is attached to the shape that was made by the corner points from step 2
Voilà, and there it is, if you would like to try it yourself you can search ‘VFX screen replacement’ on Youtube.