Hey there! i tried the new rgba brushes from 4.4.2! I am honestly extremely impressed with them, they look and behave great! The oil paint effect is very realistic! What makes RGBA brushes different and how do they work? Do they have their own engine?
They use the normal pixel engine, but use an option that was added a few versions back that allows brushtips that have an alpha channel to use the alpha directly as alpha, instead of using the grayscale value as alpha, and then interpreting the grayscale value as a lightness value. This allows making brushes that have highlights and shadows built into them. It was an option I added for making quick foliage for game artwork (using a stamp brush), but turned out to also be really effective for adding a thick paint effect to normal brushes.