In other words if you scale a 720P image into 1080P you are overlaying every 2 pixels of the original onto 3 pixels of the upscaled image. Both in the vertical and horizontal direction. A bad scaler will simply create a 50/50 blend for the middle pixel, a good one will use a more clever algorithm. (Like looking at previous and future frames to detect motion, edge detection to keep sharp edges scharp, etc) The other way around, to scale 1080P into 720P, you are trying to cram 3 pixels into 2 pixels and you have the same problem. Either way, you will get picture degradation.
The reason that upscaling from 1080P to 4K works so well is that it simply has to double the horizontal and vertical pixels, so every single pixel now becomes a group of 4. The scaling process introduces no artifacts at all. Now all the scaler has to do is decide which edges to soften or sharpen to "enhance" the image. I appreciate the effort you did but simple math would have proven the point as well.
I still have my GT750 and I use computer with a software bluray player to do the downscaling because it can do it much better than the scaler built into the projector itself. Although every person who has seen my setup is impressed at how good the image is for "just a cheap 720P", it cannot compete with a real 1080P projector when the source material is also 1080P.
Good observation regarding 1.5 ratio. It's quite obvious yet I didn't think of it before. ^^
Yes, the best scalers currently out there are implemented in madVR. Later maybe I'll do real-world shots of 720p content.
Been quiet here for a couple of months. Next week I'll be moving my GT760 outside to start making a mask of my house.
I've designed a mount and enclosure. I'm waiting for one last part to arrive to start construction. I'll take pictures along the way. The projector calculator tells me I need to be 27 ft from my house to get full coverage (52 ft wide) and be 36" below the lowest part of the house. My lawn slopes away from the house so my design should work. The projector will be about 7" off the ground.