CODEWITHSUNDEEP
phpimage-processing
Jay Are
Jay Are

Reputation: 584

Crop/cut-out image based on existing selection/path already drawn on image (ideally via PHP)

I've scoured around the internet a fair bit and I can't seem to find any reference to what I am attempting to achieve... I fear that means I'm probably going about doing something the wrong way, but I'll pose this question here anyways in hopes that maybe I am not.

I would like to take an already generated image that has a rectangular selection already drawn on it via a specific color and a dynamic (but always rectangular) path, and crop or cut-out (and use) the inner area of that rectangular path. Let's use an image generated by google maps as an example for this:Google Maps Image Example

I thought perhaps the imagemagick library would hold a solution for this, but, I don't know if it's because I haven't quite narrowed down the exact key terms for what I am looking to do exactly, or if it's because it cannot (at least not simply) be done, but I haven't turned up any solutions.

Any solutions, advice, or smacks to the head are welcome.

[Please note that (for now) I would like to operate under the assumption that these images already exist, so any information regarding the pixel coordinates of the relative selection area on the image doesn't exist]

Upvotes: 0

Views: 298

Answers (1)

r3mainer
r3mainer

Reputation: 24567

Your problem seems to boil down to this: How do I find a red rectangle in an image?

This is quite an open-ended problem, and could actually be quite difficult to solve. However, if the following assumptions can be made, then the task will be a lot easier:

  • The rectangle is drawn in pure RGB red (#ff0000).
  • The rectangle is aligned parallel with the image edges.
  • The image is saved in a lossless format like PNG.
  • The image contains no other pixels of this exact colour.
  • We know the width of the rectangle's edges.

The example you provided seems to tick all these boxes. Since it's stored as an 8-bit indexed color image, the first step would be to convert it into a true color image. This makes it easier to check the pixel values.

Then find the outermost edges of the frame, inset the coordinates by the frame width, and crop the image. Here's some code that will do this for you:

<?php

$src_img = 'er7RT.png';
$frame_color = 0xff0000;
$frame_width = 6;

// Load image and copy to true color image resource
$im = imagecreatefrompng($src_img);
$sw = imagesx($im);
$sh = imagesy($im);
$im1 = imagecreatetruecolor($sw, $sh);
imagecopy ($im1, $im, 0, 0, 0, 0, $sw, $sh);
imagedestroy($im);

// Get outer dimensions of frame.
// Assume the frame color appears nowhere else in the image.
$minx = $miny = 999999;
$maxx = $maxy = -$minx;
for ($x=0; $x<$sw; $x++) for ($y=$sh/20; $y<$sh; $y+=$sh/10) {
  if (imagecolorat($im1,$x,$y)==$frame_color) { $minx = $x; break 2; }
}
for ($x=$sw-1; $x>=0; $x--) for ($y=$sh/20; $y<$sh; $y+=$sh/10) {
  if (imagecolorat($im1,$x,$y)==$frame_color) { $maxx = $x; break 2; }
}
for ($y=0; $y<$sh; $y++) for ($x=$sw/20; $x<$sw; $x+=$sw/10) {
  if (imagecolorat($im1,$x,$y)==$frame_color) { $miny = $y; break 2; }
}
for ($y=$sh-1; $y>=0; $y--) for ($x=$sw/20; $x<$sw; $x+=$sw/10) {
  if (imagecolorat($im1,$x,$y)==$frame_color) { $maxy = $y; break 2; }
}
if ($minx>=$maxx || $miny>=$maxy) die("Couldn't locate frame");

// Subtract frame width to obtain crop region
$minx += $frame_width;
$maxx -= $frame_width;
$miny += $frame_width;
$maxy -= $frame_width;

// Create new image with cropped dimensions
$im2 = imagecreatetruecolor($maxx-$minx, $maxy-$miny);
imagecopy ($im2, $im1, 0, 0, $minx, $miny, $maxx-$minx, $maxy-$miny);

// Finish up
header("Content-Type: image/png");
imagepng($im2);
imagedestroy($im1);
imagedestroy($im2);

Upvotes: 1

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