Image resize. Require urgent help! – Coding Questions

Require urgent help! What am I doing wrong??
Maths is correct, but…

Overview:
To display a source image (example 15000 X 5000 PX) on 1920 X 1080 destination output. (keeping the correct aspect for destination)

Due to source image being > destination size, am selecting initial random positions from source image, holding the correct aspect ratio to the destination, not exceeding an output > a selected area from source. (I.e magnified) , but scaled / reduced accordingly – keeping the original aspect to destination).
Random areas from source are initially calculated and scaled down accordingly (held in an array – top left, bottom right, so to define an area from source), then displayed to destination accordingly

Issue:
Maths is fine, but something wrong with code.
Will not update selected source outputs to display, unless current window is moved via mouse. (Not updating)

Current Code:
(Please help!!)

PImage img;
PImage newimg;

int sxsize;
int sysize;

int xsize;
int ysize;

int points=100;

int[] x1=new int[points+1];
int[] x2=new int[points+1];
int[] y1=new int[points+1];
int[] y2=new int[points+1];

int xpos;
int ypos;

int distx;
int disty;

int i;

float t1;
float t2;

float aspect;

void setup(){

//fullScreen(1);
size (1920,1080);
noSmooth();
frameRate(25);

img=loadImage("1.jpg"); // use image @ 15000 X 5000 PX
sxsize=img.width;
sysize=img.height;

xsize=1920;
ysize=1080;
aspect=float(xsize)/float(ysize);

// set points
setpoints();

}

void draw(){

// get= x,y start width,height

for (i=1; i<=points; i++) {

xpos=x1[i];
ypos=y1[i];

distx=(x2[i]-x1[i]);
disty=(y2[i]-y1[i]);

newimg=img.get(xpos,ypos,distx,disty); // grab image from 1.jgp @ correct aspect ratio already calculated
newimg.resize(xsize,ysize);

image(newimg,0,0);

}

}

void setpoints(){

for (i=1; i<=points; i++) {  

// set y1,y2 in array points
 
while(true){ // find distance in y> output screen size
t1=int(random(sysize));
t2=int(random(sysize));


if(abs(t1-t2)>ysize){
break;
}
}

// if t1y2[i]){ // if t1>t2 then swap
y1[i]=int(t2);
y2[i]=int(t1);
} // end if




// set x1,x2 points

float xmaxsize=(y2[i]-y1[i])*aspect; // calc max grab width dependant on aspect ratio based upon y1,y2
t1=int(random(sxsize-xmaxsize)); // select a random area from source image less xmagesize
t2=int(((y2[i]-y1[i])*aspect))+t1; // add aspect amount to t1 so to have bottom right coordinate.

x1[i]=int(t1);
x2[i]=int(t2);

if(x1[i]>x2[i]){
x1[i]=int(t2);
x2[i]=int(t1);
}


}

}