Orthagonal X with perspective Y in OpenGL?

[deathshadow]

Got kind of a wierd situation here, Can it be done? I really only need the X orthagonal since I'm doing my own math for it, but if I could have the Y scaled to Z by OpenGL instead of having to do it myself it would really help.

Any ideas?

For those interested, I'm still working on my circlevision strip - compressing 360 degrees of X and 45 degrees of Y into screen width * 1/6th screen height.

Right now I'm using OpenGL to do this like it was Glide - not a good thing since that drastically underuses OpenGL's capabilities.

Oh, and if anyone knows a REALLY fast way to calculate Atan and sqrt, I'd love to hear it.

[cairnswm]

Cant you precalculate the ATan etc?

[pascal]For I := 1 to 360 do
MyATanArray[I] := ATan(I);[/pascal]

Etc?

[JSoftware]

The famous InvSqrt function:

[pascal]
function InvSqrt (x: single): single;
var xhalf: single;
i: integer absolute x;
begin
xhalf := 0.5*x;
i := $5f3759df - (i shr 1);
x := x*(1.5 - xhalf*x*x);
result := x;
end;[/pascal]

Original c source:

float InvSqrt (float x)
{
    float xhalf = 0.5f*x;
    int i = *(int*)&x;
    i = 0x5f3759df - (i >> 1);
    x = *(float*)&i;
    x = x*(1.5f - xhalf*x*x);
    return x;
}

WOO WORD LORD!

[User137]

Optimized (just last line off)
[pascal]function InvSqrt (x: single): single;
var xhalf: single;
i: integer absolute x;
begin
xhalf := 0.5*x;
i := $5f3759df - (i shr 1);
result := x*(1.5 - xhalf*x*x);
end;[/pascal]

[Dan]

Very interesting question with the projection...
I'm not sure how well it will work since I never needed such a wierd projection matrix, but try this:

var
  m: array[0..3, 0..3] of single;
...
  glMatrixMode(GL_PROJECTION);
  ZeroMemory(@m, SizeOf(m));
  m[0, 0] := 2;
  m[1, 1] := CoTan(FOV / 2);
  m[2, 2] := -2 / (ZFar - ZNear);
  m[3, 2] := -(ZFar + ZNear) / (ZFar - ZNear);
  m[2, 3] := -1;
  m[3, 3] := 1;
  glLoadMatrixf(@m);

[deathshadow]

Very interesting question with the projection...
I'm not sure how well it will work since I never needed such a wierd projection matrix

Hmm. Out of curiousity you wouldn't know of a projection matrix that would let me scale all 360 degrees around you into a 60 degree projection screen width? I've been doing the math for that hard-coded.

This is where I'm at:

function jangle(x,z:accuracy):longint;
var
  ta:longint;
  ax,az:accuracy;
begin
  if z=0 then begin
    if (x>0) then jangle:=JA90 else jangle:=JA270;
  end else if x=0 then begin
    if (z>0) then jangle:=0 else jangle:=JA180;
  end else begin
    ax:=abs(x);
    az:=abs(z);
    if ax>az then begin
      ta:=JA90-janglook^[trunc((az/ax)*Jadjust)];
    end else begin
      ta:=janglook^[trunc((ax/az)*JAdjust)];
    end;
    if &#40;x<0&#41; then begin
      if &#40;z<0&#41; then begin
        ta&#58;=ta+JA180;
      end else begin
        ta&#58;=JA360-ta;
      end;
    end else if &#40;z<0&#41; then begin
      ta&#58;=JA180-ta;
    end; &#123;else it's fine, leave it alone&#125;
    jangle&#58;=ta;
  end;
end;

the JangLook array is 0..jadjust, where jadjust is set to screen width * 6 (since I use the same function for both the 60 degree normal FoV, as well as the compressed 360 degree view). All those if statements mean I only need to calc the array to 45 degrees meaning I also don't have to worry too much about high angle skew errors.

I'm actually considering using this routine TWICE, once to calculate screen X off of x and z, then calculate the distance to x and z, use that distance and Y to calculate the screen y.

Basically:
screen_x:=jangle(x,z);
dis_x:=sqrt(x*x+z*z);
screen_y:=jangle(disx,y);
distance:=sqrt(dis_x*dis_x+y*y);

That means hard coding all my calculations and doing everything as a flat projection though. (which means edge detection, dual calculation for elements going off the screen sides, etc, etc)

[Dan]

Up to 180 yes, but 360... I can't think of a way to do it with one matrix