Calculating distance along a diagonal y=x in PHP

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PHP

Hi,

Imagine I have a square with a diagonal line from the bottom left to the top right corner. Given any point (x,y), I want to be able to determine how far along that line the point is. This is because I want to create a colour gradient from one colour to another, from bottom left to top right of a square.

This image displays my working here;

Given the arbitrary point (90,50), I believe I should calculate where the hypotenuse of the formed triangle begins along the square's hypotenuse. The point is directly diagonal, so it maps to position z along the square's hypotenuse.

Therefore I should be able to do the following to generate an RGB value:

$r = self::map(hypot($y, $y) + (hypot($x, $y) / 2), 0, hypot($imgW, $imgH), 0, 255);
$c = ImageColorAllocate($base_image, 0, $r, $r);

by mapping from 0 to the hypotenuse length, to 0-255.

Could anyone please help me to figure out why the output is this here[^]?

There are two issues; the gradient is completely wrong, and there are some black pixels in the bottom right which have seemingly chosen not to participate in this code.

What I have tried:

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