Digital Signal Processing Reference
In-Depth Information
7
ALPHA BLENDING
CHAPTER OUTLINE
7.1 Introduction 49
7.2 Concept and Math Behind Alpha Blending
50
7.3 Implementing Alpha Blending in Hardware
51
7.4 Creating a Different Background
51
7.5 Conclusion
52
Alpha blending, or alpha compositing, is a video-processing
technique that is used to combine two or more video streams
on the same frame. This technique is increasingly used to
overlay some graphics on a video, such as the 10-yard line in
a football match or to put a different background behind
a speaker.
We will learn how this is achieved and what kind of processing
is required to overlay images.
7.1 Introduction
We will start with two images. Video consists of frames moving
at a certain speed (30 fps or 60 fps) and we generally talk about
processing within the frame, or spatial processing. When we
process across frames it is called temporal processing. Alpha
blending is an example of spatial processing.
Each frame is made of pixels, so when we alpha blend two
frames we are blending two sets of pixels and coming up with
a new pixel in the process. How we blend the two pixels depends
on what we want to achieve.
We will start with a simple example: you want to overlay a logo
on a frame and you want the logo to appear in the left bottom
corner for each and every frame. The simple thing would be to
identify all the pixels on the original image that would be overlain
by the logo and replace those pixels with the pixels that constitute
that logo.
This is simple in principal but hard to do across video frames
that are zipping past at 60 fps. What if you did not want a simple
