Sunday, 14 December 2014

Twenty years of Skidmarks

Lately I've been digging through the old Skidmarks archives, and I came across this wee gem from 1993, no doubt written with lots of help from Simon:

Function.q hite {di.q,dj.q,oset.l}
  UNLK a4
  MOVE.l d2,a0
  MOVE.l d1,d2:SWAP d2:EXT.l d2:ASL.l #7,d2:ADD.l d2,a0
  MOVE.l d0,d2:SWAP d2:EXT.l d2:ASL.l #1,d2:ADD.l d2,a0

  MOVEM (a0)+,d2-d3 ;d0-d1 xy d2-d5 p0-p3
  LEA 124(a0),a0:MOVEM (a0)+,d4-d5
         MOVE d0,d6:MULU d1,d6:SWAP d6:MULU d6,d5  ; x. y.p3
  NOT d0:MOVE d0,d6:MULU d1,d6:SWAP d6:MULU d6,d4  ;-x. y.p2
  NOT d1:MOVE d0,d6:MULU d1,d6:SWAP d6:MULU d6,d2  ;-x.-y.p0
  NOT d0:           MULU d1,d0:SWAP d0:MULU d3,d0   ; x.-y.p1
  ADD.l d2,d0:ADD.l d4,d0:ADD.l d5,d0 ;total
End Function

For those not quite brave enough to decipher the 68000 assembly, here's what a strictly literal translation might be:

float Height(float x, float y, short *heightField){
  heightField += int(x) * 64;
  heightField += int(y);

  short height2 = *heightfield++, height3 = *heightField++;
  heightField += 62; short height4=*heightfield++, height5 = *heightField++;
  float result5 = frac( x) * frac( y) * height5;
  float result4 = frac(-x) * frac( y) * height4;
  float result2 = frac(-x) * frac(-y) * height2;
  float result0 = frac( x) * frac(-y) * height0;
  float result = (result0 + result2 + result4 + result5);
  return result / 64;

The motivation is that I've been working towards a new version for mobile devices, with a working title of "Super Skidmarks 2000" (hashtag #SS2K)

Here's what the modern version of that same function looks like, this time in C++ :

float SKTrack::GetHeight(float axisI,float axisJ)const{
  int fi=(int)floor(axisI);
  int fj=(int)floor(axisJ);

    return 0.0f;
   return 0.0f;
  int index=fi+fj*64;
  float v0=HeightField[index];
  float v1=HeightField[index+1];
  float v2=HeightField[index+64];
  float v3=HeightField[index+65];

  float s=axisI-floor(axisI);
  float t=axisJ-floor(axisJ);

  float v01=v0*(1.0f-s)+v1*s;
  float v23=v2*(1.0f-s)+v3*s;

  float height=v01*(1.0f-t)+v23*t;
  return height;

As always, any questions / comments, or suggestion for a better name etc, please leave a comment below!


  1. Thanks for posting, this might come in handy ;)

  2. These news is like heaven on earth!

    Played Skidmarks 2 WAY to much on my A1200 as a kid. That was actually one of the games that made me want to become a game programmer ( ThemePark and A-Train being 2 others ) and which I also became and have been doing for the past 15 years. so thank you for that masterpiece and I'm really looking forward to Skidmarks 2000!