Detecting islands in image with GD in PHP - php

Question

I am working on a PHP image processing library that works with rendered images from a game, being processed on a PHP webhost. In order to process the image, I need to find islands of red(where the hue is within x amount of 0 or 360), and get a point within them(not necessarily the center, but preferably near the center). There are about 100 such islands, of all different sizes. They are trapezoids or near that. Since the image is a PNG, and is uncompressed without antialiasing, the edges are crisp to the pixel, but some areas may be darker than the others.
Currently, what I have tried is using imagecolorat( in GD and testing the HSV value after conversion, followed by trying points around it until I reach a non-red pixel, but that process seems take some time to complete, and appears to encounter PHP timeouts due to the 5 second limit, if I am processing a large image. Is there a more efficient way to detect said islands and get a point not necessarily at the center, but preferable near?
I've also tried, where I know the size of the trapezoids, assuming that none come within a certain distance and skipping that distance to save time.
I don't necessarily need code, just a pointer in the right direction.
My current code:
function RGBToHSL($RGB) {
$r = 0xFF & ($RGB >> 0x10);
$g = 0xFF & ($RGB >> 0x8);
//SNIP
}
$image=imagecreatefrompng($filename);
$redislands=[];
for($xpos=0; $xpos<=imagesx($image); $xpos++){
for($ypos=0; $ypos<=imagesy($image); $ypos++){
if (RGBToHSL(imagecolorat(xpos, ypos)->saturation<=20||RGBToHSL(imagecolorat(xpos, ypos)->saturation>=350){
$redislands[]=[xpos, ypos]
}
}
}

Answer 1

I haven't really ever used gd, but I just looked briefly and I see
imagefilter() w/ the IMG_FILTER_EDGEDETECT flag.
seems like after that, you could then identify all the objects with a single simple loop(if objects have a min size, you can make the loop use this stripe size and still be guaranteed you'll encounter each object). Just look for a pixel w/ the edge color, and when you find one, recursively explore neighbor pixels w/ the edge color. For example, if an object is guaranteed to be at least 7 pixels high, after edges are highlighted, you only need to loop over the pixels in rows 0, 7, 14, 21 etc...
Once you've extracted the component by using the edge around it, I think you could identify it's shape, color, and center fairly easily.
You might need to play with thresholding the image(gd can do this) if edges don't get detected robustly.
Another option to seriously consider is to not use php for this, but instead execute an external program using php's exec() function. This opens up the possibility of using all kinds of libraries(like opencv) and programs meant for this stuff. It also lets you write your code in something like c...you'd be amazed at how fast you can loop over a c array that holds a few million ints representing pixel colors. But, I have a feeling this isn't a viable option for you.
Something else you might investigate is getting access to a php array(or a string) of pixels. function call overhead is large, and calling a function like imagecolorat() millions of times does add up, compared to native php array access like $pixels[$i]. I don't know anything about image formats, but maybe you could save as bitmap and then load it into a string, and then just use string offsets. I bet that would fly if a bitmap has a suitable array-like binary representation.