Compositing Fun with Additive Blends!

Posted by Walter

I’m happy to announce that we’ve added additive blend support to Corona. It’s available starting in Corona daily build #497.

What’s additive blends? It’s a cool way to get glow effects. It’s also known as “Linear Dodge” for the Photoshop buffs among you.

In the rest of this post, I’ll break it down into its gory details with code, math, and pictures.

Whenever you’re doing alpha blends between two different objects, you’re doing compositing. By default, Corona does the standard “normal” blend which is what most of us think intuitively think of when we say blend two objects together, i.e. the painter’s algorithm.

So if you use the following code, which places a transparent PNG on top of a background…

…You’d get:

For the math buffs among you, the equation for normal blends looks like this:

colorFinal = (colorTop)*(alphaTop) + (colorBottom)*(1 – alphaTop)

Now, there are a ton of possible blend modes. Photoshop has around 2 dozen. It turns out that there’s another way to do blends which is incredibly useful. That’s the additive blend mode (a.k.a. linear dodge for you Photoshop types). This blend mode merely adds the pixel values of two overlapping objects on a per-channel basis (up until saturation):

The blending equation here looks (if the sum exceeds saturation, then the channel just takes on the max value):

colorFinal = (colorTop)*(alphaTop) + (colorBottom)

To do this, you just need to set a new “blendMode” property of the display object:

The other value for blendMode is “normal” which is the default.

There are a couple of limitations. This only works on vector and image objects. This does not work on groups. And we’re not supporting fancy things like using masks simultaneously (that will have to wait until we introduce shaders), so your mileage may vary.

Special thanks to Joe Kauffman for sharing some image files for me to use. You can also thank him for guilting me into adding this feature. I know he and several others have been waiting awhile, so I finally found some free time to implement it.