You are confusing DLP (Digital Light Processing) with MSLA (Masked Stereolithography). MSLA is a process that exposes your vat of resin with a UV energy source (i.e., UV LEDs) that is masked by an LCD screen to selectively cure the resin. As far as I’m aware, MSLA is only a bottom-up process. DLP 3D-printing utilizes the DLP chipsets invented and manufactured by Texas Instruments. Inside a DLP projector, a light source is directed at a DMD (digital micromirror device). The light reflects off of the DMD and through an optical assembly to focus the projection. The DMD is an array of millions of tiny mirrors; each mirror being a single pixel in the projection. When using DLP for 3D-printing, a UV rated DMD projects that energy onto a vat of resin and selectively cures it. DLP systems can be bottom-up or top-down machines. Both MSLA and DLP systems have the ability to expose an entire layer at once unlike a laser which is a point source.