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;

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!