The Compound Blur effect blurs pixels in the effect layer based on the luminance values of a control layer, also known as a blur layer or blurring map. So we have here some footage, and we also have this ramp going from black to white. Apply the Compound Blur by going to Effect, Blur & Sharpen, Compound Blur. By default, the effect will use your footage as a blur map. Brighter areas will have more blur, and darker areas have less. You can select Invert Blur to swap that.
Instead of using our footage, we want to use our blur map. So next to Blur Layer, select your ramp layer. So you can see that we fade from no blur, where the ramp was completely black, to the maximum blur, where the ramp was completely white. Changing the Maximum Blur value will change how much the footage is blurred where the ramp is completely white.
If you take a look at my ramp here, you can see that it is not the same size as my composition. The Stretch Map to Fit checkbox will…you know…stretch the map so that the effect covers the entire comp. You can uncheck that box if you don’t want to stretch the map.