flipping lines in 3D

Programming Questions

allonestr..

in Programming Questions • 2 years ago

Playing with spheres and stumbled onto this. The answer doesn't really matter, but why do the lines flip over?

string

//original from matt pearson's genart
import processing.opengl.*;
int radius = 150;
float betaInc = PI/10;
float gammaInc = PI/360;
void setup()
{
size(400, 400, OPENGL);
background(255);
frameRate(12);
}
void draw()
{
background(255);

translate(width / 2, height / 2, 0);
rotateX(frameCount * 0.04);
rotateY(frameCount * 0.03);
rotateZ(frameCount * 0.01);

float beta = 0;
float gamma = 0;
float lastx = 0;
float lasty = 0;
float lastz = 0;

while(gamma < PI)
{
    beta += betaInc;
    gamma += gammaInc;

    float thisx = radius * cos(beta) * sin(gamma);
    float thisy = radius * sin(beta) * sin(gamma);
    float thisz = radius * cos(gamma);

    strokeWeight(sin(gamma) * radius / 5);
    float colourVal = map(thisz, -radius, radius, 0, 255);
    stroke(colourVal, colourVal / 2, 255 - colourVal);
    if(lastx != 0)
    {
      line(lastx, lasty, lastz, thisx, thisy, thisz);
    }

    beta += betaInc;
    gamma += gammaInc;

    float nextx = radius * cos(beta) * sin(gamma);
    float nexty = radius * sin(beta) * sin(gamma);
    float nextz = radius * cos(gamma);
    lastx = nextx;
    lasty = nexty;
    lastz = nextz;
}
}

Replies(3)

kooogy

can't see anything in the code that would cause that. but i have a theory it's caused by the stroke width and line()

i'm guessing the line is drawn as a rectangle, with no depth. and there's someting in processing or opengl library that orientates this line so that it faces the camera ("billboarding") otherwise there'd be times when it's edge on and invisible. only that orientation's not done too well, or is limited to the three axes, so that at certain times it'll flip from one axis to another.

using P3D gives, i think, a different effect, the flipping isn't as sudden and there seem to be intermediate steps. or it might just be that P3D is slower so i can see that clearer.

allonestr..

Re: flipping lines in 3D

2 years ago

I've been staring at it and am wondering too if the line is drawn with its "top" facing the screen (that is, me), and when I rotate it, it turns so that that "top" face still faces me.

I noticed that P3D wasn't so elegant.

An ellipse behaves differently, always lying on the xy-plane.

Much experimenting needed - there's this afternoon gone!

Here's a cube ...

import processing.opengl.*;
float side = 50;
int bit = 20;
float weight = 30;
void setup()
{
size(300, 300, OPENGL);
background(32);
}
void draw()
{
background(32);

translate(width / 2, height / 2, 0);
rotateX(frameCount * 0.001);
rotateY(frameCount * 0.002);
rotateZ(frameCount * 0.003);

drawLines(-side, -side, -side);
drawLines(side, side, -side);
drawLines(-side, side, side);
drawLines(side, -side, side);
}
void drawLines(float x, float y, float z)
{
float nx, ny, nz;
if(x > 0) nx = x - bit;
else nx = x + bit;
if(y > 0) ny = y - bit;
else ny = y + bit;
if(z > 0) nz = z - bit;
else nz = z + bit;
strokeWeight(weight/2);
stroke(255, 128);
line(nx, y, z, -nx, y, z);
line(x, ny, z, x, -ny, z);
stroke(255, 0, 0, 128);
line(x, y, nz, x, y, -nz);

for(int i = 0; i <= bit; i += 1)
{
    strokeWeight((weight / bit) * i);
    float ix, iy, iz, ix1, iy1, iz1;

    if(x > 0) {ix = x - i; ix1 = x - i + 1;}
    else {ix = x + i; ix1 = x + i - 1;}
    if(y > 0) {iy = y - i; iy1 = y - i + 1;}
    else {iy = y + i; iy1 = y + i - 1;}
    if(z > 0) {iz = z - i; iz1 = z - i + 1;}
    else {iz = z + i; iz1 = z + i - 1;}
    stroke(255, 128);
    line(ix, y, z, ix1, y, z);
    line(-ix, y, z, -ix1, y, z);
    line(x, iy, z, x, iy1, z);
    line(x, -iy, z, x, -iy1, z);
    stroke(255, 0, 0, 128);
    line(x, y, iz, x, y, iz1);
    line(x, y, -iz, x, y, -iz1);
}
}

allonestr..

Re: flipping lines in 3D

2 years ago

Here's a "better" version of that cube.

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