Not able to apply alpha to points in Processing.org sketch - processing

Question

I am trying to change the alpha value of individual points. I set an alpha value using the stroke() method and I've tried other methods like setting it using fill() even though that doesn't really apply to points, but I'm still not able to change the alpha value.
Steps to reproduce:
Here's a short code example. Two red lines. One has the default alpha (255) and one has alpha=100. I have a blue line crossing them with alpha=100.
void setup()
{
size(500, 500);
background(255);
colorMode(RGB);
smooth();
}
void draw() {
int x = 250;
int y = 250;
int alphaval = 100;
strokeWeight(5);
for (int del = 0; del < 100; del = del+1) {
stroke(255, 0, 0);
point(x-20-del/2, y-del/2);
}
for (int del = 0; del < 100; del = del+1) {
stroke(255, 0, 0, alphaval);
point(x-del/2, y-del/2);
}
for (int del = 0; del < 100; del = del+1) {
stroke(0, 0, 255, alphaval);
point(x-60+del/2, y-del/2);
}
}

Answer 1

Ah ha! I found out from someone that I had forgotten to put background(255) at the start of my draw() method so I was drawing the points repeatedly over and over which cancelled out any effect of alpha. This shows the alpha working:
void setup()
{
size(500, 500);
background(255);
colorMode(RGB);
smooth();
}
void draw() {
background(255);
int x = 250;
int y = 250;
int alphaval = 10;
strokeWeight(20);
for (int del = 0; del < 100; del = del+1) {
stroke(255, 0, 0);
point(x-30-del/2, y-del/2);
}
for (int del = 0; del < 100; del = del+1) {
stroke(255, 0, 0, alphaval);
point(x-del/2, y-del/2);
}
for (int del = 0; del < 100; del = del+1) {
stroke(0, 0, 255, alphaval);
point(x-60+del/2, y-del/2);
}
}