Shader: How to get a greyscale screen texture with the pixels as emissive areas

Hey everyone, I'm trying to work my way into shaders these days and I've been thinking about a bloom shader as an Image Effect. I thought: well, I can grab the current frame and a 'mask' with all the areas that emit lights, and then do some math to blur the mask and add a glow where the mask isn't fully black. The question is: Is it even possible to get a 2D mask (from the whole screen), with white at the fragment where an emission map exists and is emitting, and black where a fragment isn't emitting. I don't think this is possible, but I'd wanted to check it anyways. Thanks!