Question

I have the following php code:

<?php

$image = imagecreatefrompng("captcha_background.png");

$imgcolor = imagecolorallocate($image, 0, 0, 0);

imagesetthickness($image, 2);

imageline($image, 0, 25, 40, 90, $imgcolor);

?>

The method “imageline” draws a straight line on my image from the coordinates 0 (x) 25 (y) to 40 (x) 90 (y).

The result is the following image:

What I’m confused about is the reverse of the bottom and the top when using coordinate systems in php.
Normally 0 (The starting point) would be in the lower left corner, but when assigning coordinates in the method “imageline” the 0 (Starting point) is located in the upper left corner?

Expected result:

(The image is 300×100 pixels)

Could someone please explain why this is happening?

Answer 1

This is not a mathematical graph. The typical coordinate system used in development (as far as I know) is to have the first quadrant at the lower right. That is, 0x0 is at the top left. This applies to all html elements that have widths and heights (the elements drop down, they do not fall up).

The motivation appears to be the fact that it’s hard to tell how much height you have to work with without knowing the absolute height of the image, which you may not know at any given time, and which may change frequently.