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

  LSR.l#6,d0:RTS

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){

  //__asm{...}

  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);

  if(fi<0||fi>=63){

    return 0.0f;

  }

  if(fj<0||fj>=63){

   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!

Pantsfree Programming

A good friend of mine (who shall remain nameless) has a great expression: "Pantsfree programming."

It took me a little while to figure out what he meant.

Pants

You see, I spent a lot of time in England where "pants" can be used as a mild swear word.  A bit like how the Americans would say "*!*?", or perhaps "!?*!".

Eventually I figured out he meant "Programming without trousers."

But why do we all need trousers to program a computer? What catastrophe would befall us if we were to take them off?  I was so confused.

But working through it a little more, I finally figured out he meant "Programming without shame."

Fearless

It's the programming you do when you don't care what anyone else thinks.  It's fearless programming.  It's liberating. It's the programming you do when the journey is everything, and the destination is almost a distraction.  There's no intention to finish, and you never need to show your working or your results.

But there's a reason you're using your trouser-free time for programming rather than playing video games.

I think we all need a little bit of "Pantsfree programming" in our lives.  And maybe we could all make use of some of those principles in other parts of our lives too.

If you're "Pantsfree" right now, please leave a comment!