If it’s a PC game I don’t understand why it would even need an upscale filter why can’t they just slightly modify it to actually render in a high res resolution? I mean if freeware super Nintendo emulators can do it…
StarCraft’s Sprite based, So the original sprites included in the game were made with the assumption that it’s playing at 640*480. Before 1.18, the game would actually force your monitor to display at that resolution, meaning that it’d use the monitor’s upscale tools. Now, in the recent versions, upscaling is handled by the program, so it uses its own upscaling and smoothing tools.
With sprites, you can’t add detail that wasn’t there to begin with when projecting at a higher resolution. The Super Nintendo (assuming SNES) emulator’s tools for HD display are filters to smooth it out based on guesswork. In order to truly display a Sprite based game in HD, you need HD Sprites, which is why StarCraft: Remastered was made.