imageantialias() doesn't seem to do anything on circles - php

Question

I create an image with:
$image = #imagecreatetruecolor(320, 45) or die("Cannot Initialize new GD image stream.");
imageantialias($image, true);
imagefill($image, 0, 0, imagecolorallocate($image, 255, 255, 255);
Later, I add some text and on top of that, a shape:
imagearc($image, $x + 5, $y - 7, $size, $size, 0, 360, $color);
But the the circle is still very jaggy:
I'm running php 8 with gd installed. I'm getting no errors.

Answer 1

Ok, this guy has a whole web page devoted to algorithms to work around this limitation: https://create.stephan-brumme.com/antialiased-circle/
Awesome work.
I went with "Wu's Algorithm" which provides the best quality. Works well.
Here's my version modified to use my global $image variable. It creates a perfect circle. The $my_x and $my_y are the x and y coordinates on the canvas.
function create_circle($my_x, $my_y, $size, $color) {
global $image;
$width = $size;
$height = $size;
$centerX = $my_x; $radiusX = ($width -20) / 2;
$centerY = $my_y; $radiusY = ($height-20) / 2;
static $maxTransparency = 0x7F; // 127
$radiusX2 = $radiusX * $radiusX;
$radiusY2 = $radiusY * $radiusY;
// upper and lower halves
$quarter = round($radiusX2 / sqrt($radiusX2 + $radiusY2));
for ($x = 0; $x <= $quarter; $x++) {
$y = $radiusY * sqrt(1-$x*$x/$radiusX2);
$error = $y - floor($y);
$transparency = round($error * $maxTransparency);
$alpha = $color | ($transparency << 24);
$alpha2 = $color | (($maxTransparency - $transparency) << 24);
setpixel4($centerX, $centerY, $x, floor($y), $alpha);
setpixel4($centerX, $centerY, $x, floor($y)+1, $alpha2);
}
// right and left halves
$quarter = round($radiusY2 / sqrt($radiusX2 + $radiusY2));
for ($y = 0; $y <= $quarter; $y++) {
$x = $radiusX * sqrt(1-$y*$y/$radiusY2);
$error = $x - floor($x);
$transparency = round($error * $maxTransparency);
$alpha = $color | ($transparency << 24);
$alpha2 = $color | (($maxTransparency - $transparency) << 24);
setpixel4($centerX, $centerY, floor($x), $y, $alpha);
setpixel4($centerX, $centerY, floor($x)+1, $y, $alpha2);
}
}
// helper function, draws pixel and mirrors it
function setpixel4($centerX, $centerY, $deltaX, $deltaY, $alpha) {
global $image;
imagesetpixel($image, $centerX + $deltaX, $centerY + $deltaY, $alpha);
imagesetpixel($image, $centerX - $deltaX, $centerY + $deltaY, $alpha);
imagesetpixel($image, $centerX + $deltaX, $centerY - $deltaY, $alpha);
imagesetpixel($image, $centerX - $deltaX, $centerY - $deltaY, $alpha);
}