I thought a little bit more about my idea, and I came up with a version that doesn't need any data reads or pixel comparisons.
So we would start with the same thing: no screen clearing and the previous frame is always preserved.
But when drawing the next frame, we only draw every other column. So if screen has 320 columns, we only update 160 of them. This results in a screen where every other column is new and every other is from the previous frame. And when the next frame arrives then we change the draw order: if in previous frame we drew columns 1, 3 and 5 then in this frame we draw columns 2, 4 and 6.
So this method would surely be faster: no screen clearing, and only need to raycast half of the columns each frame.
However, I'm not sure if this would look good. But because every other column would be 100% right, and the other half would be "approximately right", then maybe it could look acceptable?
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Also, a more advanced version of this could be something like this: we draw every column, but skip every other pixel in the columns.
So in each column only every other pixel would be updated, and the rest would be from the previous frame. And for the next column we change the order, so if in the current column we updated pixels 1, 3 and 5 then in the next column we update pixels 2, 4 and 6.
So the resulting screen would have a "chessboard pattern" of new and old pixels. And when the frame changes, then we switch the pixel updating order in the columns, so that the old and new pixels alternate every frame.
But again, would this look good? Don't know.
