So, you never had an Oric-1, did you
ROM 1.0 has a display bug, showing the TEXT status line in the middle of the HIRES screen, thus damaging it. What you see here is actually "Saving .." in hires ! That means Macadam Bumper was created on Oric-1 (the RTS in #9FFF also shows that, if you don't do that in ROM 1.0, CLOADing a HIRES screen in a basic program will go back to "Ready").
This bug is corrected in ROM1.1. That's why HRC won't work on Oric-1 from tape: "Loading .." will crush the compressed data.
My opinion is that initially, the "hidden line" between HIRES and the 3 TEXT lines was supposed to actually hide this status line on screen when in HIRES (so the 3 TEXT lines would be the TEXT upper lines), but the RAM MAP in HIRES mode must have been modified, and they couldn't change the ULA anymore. Maybe I'm totally wrong but I can't explain this hidden line in any other way.
Back to Macadam Bumper, they later released an updated version with a black attribute to hide the garbage (but they did not correct the drawing, so there was still a black line).
Mmmh, doable but limiting pictures a lot! Maybe it would be simplier to used another RAM area to load the compressed data if you need to hide it.Else I was thinking if we can hide somehow the compressed garbage being displayed before the real image and maybe it's theoretically doable:
- source image should be very simple to allow high compression ratio;
- first two bytes in every compressed line are reserved to represent same color (for instance PAPER0:INK0), followed by 38 bytes real compressed data;
- the compressed bytes will use only 6 bits (i.e 01xx xxxx - all attribute codes are prohibited);
- the decompression loop will be more complex but must be 40 bytes max...
It seams possible but is it worth?
Because here, even if you hide it under black attributes, if there was another HIRES picture on screen before, you will see it partially erased before the next one decompresses.
I guess it all depends on what you need to do