AGI, SCI, and combined priority/control screens

This post is dedicated to Cameron.

Last post, I ended with this claim:

This has been the case all the way since AGI.

It’s basically true, but there are some interesting details about AGI’s priority screen. For starters, it’s also the control screen.

Any color over a particular number is considered priority, while the lowest few are control. Thus, black is blocking, green is trigger, and blue is water. But if the control lines are drawn on top of the priority info, how do you not get unsightly gaps? If Gwydion were standing behind that table, wouldn’t you see his legs through that black gap? Turns out no, you wouldn’t. For lack of AGI source, here’s a part of ScummVM:

bool GfxMgr::checkControlPixel(int16 x, int16 y, byte viewPriority) {
  int offset = y * SCRIPT_WIDTH + x;
  byte curPriority;
 
  while (1) {
    y++;
    offset += SCRIPT_WIDTH;
    if (y >= SCRIPT_HEIGHT) {
      // end of screen, nothing but control pixels found
      return true; // draw view pixel
    }
    curPriority = _priorityScreen[offset];
    if (curPriority > 2) // valid priority found?
      break;
  }
  if (curPriority <= viewPriority)
    return true; // view priority is higher, draw
  return false; // view priority is lower, don't draw
}

In plain English, that means that when determining the priority of a given background pixel, if that pixel is a control color, you scan down to the next valid color:

But wait, this introduces errors! There are gaps in the seat and wall! And you know what? This works out fine because you can’t actually get to those points and be standing on a lower priority band. It’s all sneaky design in the end.

In SCI0, the control screen was split off from the priority screen. Black became the default value, white meant blocking, and all the others meant whatever the room programmers wanted them to mean. In a room with water, blue was the obvious choice but in a dry room blue might as well be a trigger. If something wasn’t a trigger, it was the hotspot for non-squarish background features.

In SCI1, vectored visual screens were deprecated. Instead, the background was basically a single Draw Bitmap command, followed by vector commands tracing the priority and control screens.

In SCI11, the control screen was deprecated — it was still available, but hardly used if at all. Walkable areas were now denoted with polygons, as were feature hotspots. Trigger areas were either polygons or IsInRect checks, but the priority screen worked the same as always. Priority screens were still vector traces, though.

It wasn’t until SCI2 that the system would radically change again, dropping the control screen and vectors altogether. Instead, the priority screen would be drawn at the same time as the visual screen: piece by piece.