The math of images

adaykin

Hey, I have been experimenting a little with PHP's GD library and I don't quite understand how space is allocated for a rectangle. In this example here:

<?
 $im = ImageCreate(200,200);
 $white = ImageColorAllocate($im,0xFF,0xFF,0xFF);
 $black = ImageColorAllocate($im,0x00,0x00,0x00);
 ImageFilledRectangle($im,50,50,150,150,$black);
 header('Content-Type: image/png');
 ImagePNG($im);
?>

The width and length of the image is twice as x coord - y coord. But in another image I am making:

	$im = ImageCreate(900, 900);
	$white = imagecolorallocate($im, 255, 255, 255);
	imagefill($im, 0, 0, $white);	
	$black = imagecolorallocate($im, 50, 50, 50);
	imagerectangle($im, 500, 10, 850, 360, $black);
	header("Content-type: image/jpeg");
	imagejpeg($im);
	imagedestroy($im);

I had to make the image this big so it would actually show up. Why do I need to make the image so big here? Is it because I am creating the image farther along the x axis?

Weedpacket

How big is it supposed to be? You certainly wouldn't see a rectangle with its top-left corner at (500,10) in an image that's only 200x200.

adaykin

My length for the 2nd rectangle was 900 x 900. The other rectangle which I think is what you are confusing it with, is 200 x 200. My question is isn't the gd library very inefficient if you have to specify an image length that is 900 x 900, for an image that is obviously much smaller than that. If you try my 2nd example, you will notice that if you right click to the left of the rectangle, your browser will still have the save image menu enabled.

Weedpacket

adaykin wrote:
for an image that is obviously much smaller than that.

It isn't "obviously much smaller than that" for me; in part because, unlike you, I don't know what you're trying to achieve. All I know is: you create a 900x900 image and you draw a rectangle on it.

See previous reply for my question.

adaykin

Well let me ask you this...try my second example, and the try right clicking to the left of the rectangle. Do you notice that even though you are not on the actual rectangle, you still get the menu that asks you to save an image?

madwormer2

That's because you told it to.

$im = ImageCreate(900, 900);

This line of code tells it to create an image of dimensions 900 x 900 pixels

If you know how big you want your final image to be, set the dimensions when it is first created.

adaykin

True enough. But the point is that if you make the dimensions any smaller the entire image won't show up. I obviously don't want a 900 x 900 image. Try the image with dimensions 600 x 600, and the entire image won't show up. Think about it in terms of geometry.

madwormer2

You're drawing the rectangle from point (500,10) to point (850,360)

Anything less than 850 across isn't going to contain the rectangle.

adaykin

Ok that was what I was thinking, but the problem is then you get all that extra white space that is part of your image. It just seems really inefficient to draw with a a upper left, and lower right for a rectangle, and then specify the length. Some other web graphic toolkits like dojo got it right, where you just specify one of the points, and then specify the width and height of the rectangle.

madwormer2

Yuhuh...

$x = 500;
$y = 10;
$width = 350;
$height = 350;
    imagerectangle($im, $x, $y, $x + $width, $y  + height, $black);