vertically justify the created pattern - php

Question

I have used PHP-Imagick to create a patterned-image of repeated circles, like the given example-image above. Below is the code that created the image above. I have explicitly commented the code and the variable names are kept pretty verbose in order to know whats going on.
$canvas = new Imagick();
$cw = 700; // user provided width
$ch = 300; //user provided height
$hrzntl_c = 10; //user provided - number of horizontal circles - min 2 and max 200
$c_diamtr = $cw / $hrzntl_c; //set the diameter of circle
$c_radius = $c_diamtr /2; //set the radius of circle
$vrtcl_c = $ch / $c_diamtr; //calculated number of vertical circles.
$canvas->newImage( $cw, $ch, new ImagickPixel( "seagreen" ) ); //create a canvas
$draw = new ImagickDraw(); // create a draw object
$draw->setFillColor( new ImagickPixel( "white" ) ); // set the fill color
for ($i = 0; $i <= $hrzntl_c; $i++){ //loop horizontal
for ($j = 0; $j <= $vrtcl_c; $j++){ //loop vertical
$draw->ellipse( $c_radius + $c_diamtr * $i, $c_radius + $c_diamtr * $j, $c_radius, $c_radius, 0, 360 );
}
}
$canvas->drawImage( $draw ); // render the circles to the canvas
$canvas->setImageFormat( "png" ); // set the image format to png
header("Content-Type: image/png"); // Output the image
echo $canvas;
ISSUE : Now you have got an idea that the circles are designed to be horizontally justified. I need help to justify the vertical circles as well. Look at the bottom of the example image and you can see that circles are indeed not justified. By "justified", I mean if you see a half/full circle towards top then you should see half/full a circle at the bottom too. Like this perfect example.

Answer 1

Calculate an offset based on the image height and the circle count. Btw, you should adapt your count calculation:
$vrtcl_c = ceil($ch / $c_diamtr);
$verticalOffset = ($ch - $vrtcl_c * c_diamtr) / 2;
Then, shift your drawings:
for ($j = 0; $j < $vrtcl_c; $j++){ //loop vertical
$draw->ellipse( $c_radius + $c_diamtr * $i,
$c_radius + $c_diamtr * $j + $verticalOffset,
$c_radius, $c_radius, 0, 360 );
}