Faster Mode-X Sprite Rendering

Back in June, I made a sprite rendering system for DOS/VGA Mode-X.  Running it on a pcem 386 showed that the sprite clipping code was hella slow.

So I decided to rip out the clipping code.  To allow sprites to overlap the screen boundary, I set up the VGA display registers to have a 32px off-screen border around the display.

The system still splits the sprite into separate planes, and it still works by skipping some pixels then drawing some.

It works across a scanline (within a single plane) so I can use rep movsb.  It also hard-codes the scanline width into the number of pixel bytes to skip - that's so I don't have to multiply the display width by the number of scan-lines to skip.  That means my game can only run in a single screen resolution, but it's only going to run in Mode-X so that's not a problem.

Another tweak is that as much as possible, it loads 16-bits of data at a time.  This should make better use of the bus, which can be an issue with the 386SX.

The command buffer format:

  • number of skip/draw pairs (word) - used as a loop counter
  • for each skip/draw pair:
    • numToSkip/numToDraw (word)
      • top 3 bits are numToDraw-1 (min 1, max 8)
      • bottom 13 bits are numToSkip (can include multiple scan-lines)
    • numToDraw * pixel bytes
    • 0 or 1 bytes to pad to 2-byte boundary

The asm code to render a plane is this:

DrawSkipDrawPlane PROC near WATCOM_C public   
      push ds  
      push si  
      push es  
      push di  
      push cx  
   
      mov ds, ax     ; ax holds command segment  
      mov si, dx     ; dx holds command offset  
      mov di, bx     ; bx holds destination offset  
   
      mov cx, 0xA000  
      mov es, cx     ; set up vga segment  
   
      lodsw          ; load number of skip/draw pairs  
      mov dx, ax     ; ...into dx  
        
      skipDrawLoop:  
           lodsw     ; load numDraw/numSkip  
             
           mov cx, ax  
           shr cx, 13     ; split out (numDraw-1) into cx  
           inc cx  
             
           and ax, 0x1fff     ; mask to get numSkip  
           add di, ax          ; skip them  
             
           rep movsb     ; mov cx bytes of pixel data from command list to screen  
             
           inc si  
           and si, 0xfffe     ; round si up to 2-byte boundary  
             
           dec dx                    ; another skip/draw pair?  
           jnz     skipDrawLoop     ; ...then go do it  
        
      pop cx  
      pop di  
      pop es  
      pop si  
      pop ds  
        
      ret  
 DrawSkipDrawPlane ENDP  



Comments

Post a Comment

Popular posts from this blog

Micro:Bit and SPI display

I want to explore the full potential of the BBC Micro:bits my school has. Making rock-paper-scissors games and the like is all well and good, and the students love it (for the most part), but it always feels a little bit gimmicky.  But when students really get engaged and suggest ideas for them (e.g. games, compass display), it all gets a little bit cramped and feels like it could  kinda  work, but not really . Then I found out that all the tiny strips on the edge are actually GPIO pins, not just the fat 0, 1 and 2 ones.  Since then, I've wanted to explore what I can do with it. Hence, over the last few days I have been trying to get the microbit to control a TFT display. I first went for this 2.4" display with a touchscreen.  The SPI interface is just for the microSD, not for the screen but I didn't think that would be a problem because the micro:bit has so many pins.  After much frustration, I discovered that there are many of the pins you can't reliabl...
Read more

DCS World with TrackIR under Ubuntu

This is a record of my experience of setting this up.  It's meant more as a record for my future self than a tutorial, but maybe somebody will find it useful. 1 - Install DCS First step, install DCS World via the Steam client as normal.  2 - Install LinuxTrack Linuxtrack Download here: https://github.com/uglyDwarf/linuxtrack/wiki/Downloads   Instructions here: https://github.com/uglyDwarf/linuxtrack/wiki/universal-Linuxtrack-package   Using the gui (ltr_gui) requires qt4 libraries, which I could not find in my standard Ubuntu repositories, so I had to use the ppa instructions here: https://ubuntuhandbook.org/index.php/2020/07/install-qt4-ubuntu-20-04 Run ltr_gui from linuxtrack's bin directory to check everything works so far before moving on to get it running under Proton.  3 - Install LinuxTrack into the DCS World prefix NOTE: Attempting to install for Wine games WON'T WORK e.g. here https://github-wiki-see.page/m/uglyDwarf/linuxtrack/wiki/Wine-Games   Th...
Read more

Polygon Triangulator!!

For my 3D games, I've been playing around with ideas around semi-procedural geometry, by which I mean geometry which is generated from a small amount of authored data.  For example, I could draw out the shape of an island in a dozen or so 3D points, and the actual terrain geometry could be generated from that. A big step along the way to that is splitting up a non-convex polygon into triangles.  The triangles mustn't overlap, nor must they cover any space which the polygon itself doesn't cover.  For example, this diagram . Today, I managed to get a python program* running which took a list of 2D points defining a polygon, split it into sub-triangles and displayed them in a tkinter canvas**. I am certain this has been solved by clever people in much better ways, but I still feel proud to have figured this one out (and I haven't managed to break it yet!). The basic algorithm is: Create a temporary copy of the polygon points list start with the three points fro...
Read more