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The effect of coordinate order on closure effect in imageopenpolygon()

M66 2025-06-02

When using PHP to process images, imageopenpolygon() is a very practical function that can draw open polygon paths (i.e., head and tail are not automatically connected). However, many friends may be puzzled when using it:

In order to figure out this problem, we have done detailed tests. Let’s take a look together below.

What is imageopenpolygon()

In PHP's GD library, imageopenpolygon() is used to draw open polygon lines. Unlike imagepolygon() , it does not automatically connect the first point and the last point.

Basic usage examples:

 <?php
// Create a canvas
$image = imagecreatetruecolor(400, 400);

// Set background color to white
$white = imagecolorallocate($image, 255, 255, 255);
imagefill($image, 0, 0, $white);

// Set the line color to blue
$blue = imagecolorallocate($image, 0, 0, 255);

// Define a point array（Clockwise）
$points = [
    50, 50,  // The first point
    150, 50, // The second point
    150, 150,// The third point
    50, 150  // The fourth point
];

// Draw open polygons
imageopenpolygon($image, $points, count($points) / 2, $blue);

// Output picture
header('Content-Type: image/png');
imagepng($image);
imagedestroy($image);
?>

After running this code, you will see a U-shaped open quadrilateral, where there is no connection between the starting point and the end point .

Change the coordinate order to see the effect

Next we adjust the order of points to counterclockwise :

 $points = [
    50, 50,  // The first point
    50, 150, // The second point
    150, 150,// The third point
    150, 50  // The fourth point
];

Run again, the drawn shape is still open, except that the lines move differently .

Summary 1 :

  • The coordinate order does not cause the figure to close .

  • The coordinate order only affects the drawing order of the lines (that is, the direction of the lines will change).

What if I want to close manually?

If you want the figure to look closed, you need to manually add the first point to the end of the array, for example:

 $points = [
    50, 50,
    150, 50,
    150, 150,
    50, 150,
    50, 50 // 再加一次The first point
];

In this way, the figure drawn by imageopenpolygon() will be connected to the end, and it looks like a closed polygon!

Sample diagram preview

To facilitate viewing different effects, here is a sample image address:

(If you want to generate it yourself, you can also try it with the above PHP code directly.)

summary

  • imageopenpolygon() is not closed by default, regardless of coordinate order.

  • If you need to close, you need to add the first point at the end .

  • The coordinate order affects the direction of line drawing , not closed behavior.

I hope this practical explanation can help you quickly understand and master the small details of imageopenpolygon() in actual development!

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