Generated image with text always off-center

Question

I have a simple PHP avatar generator that takes someone's username, and makes an image with their initials on it. This works well, but somehow the text is always off center and I cannot figure out have to always have the text properly centered.

This uses the Ubuntu font.

$im = imagecreatetruecolor(125, 125);

$str = 'dpoint';
if (str_word_count($str) > 1)
{
    $words = explode(' ', $str);
    $text = $words[0][0]. $words[1][0];
}
else
{
    $text = substr($str, 0, 2);
}

$font_size = '50';
$white = imagecolorallocate($im, 255, 255, 255);
$angle = 0;

// Get image dimensions
$width = imagesx($im);
$height = imagesy($im);
// Get center coordinates of image
$centerX = $width / 2;
$centerY = $height / 2;
// Get size of text
list($left, $bottom, $right, , , $top) = imageftbbox($font_size, $angle, './Ubuntu-L.ttf', $text);
// Determine offset of text
$left_offset = ($right - $left) / 2;
$top_offset = ($bottom - $top) / 2;
// Generate coordinates
$x = $centerX - $left_offset;
$y = $centerY + $top_offset;
// Add text to image
imagettftext($im, $font_size, $angle, $x, $y, $white, './Ubuntu-L.ttf', $text);

// Set the content type header
header('Content-Type: image/jpeg');

// Skip the file parameter using NULL, then set the quality
imagejpeg($im, NULL, 100);

// Free up memory
imagedestroy($im);

That for example ends up like this: You can see it's much lower down that it should be. What positioning am I doing wrong here?

Ideally I just want it so no matter the text size or lower-case or upper-case, it just centers it vertically and horizontally properly inside the box.

Answer 1

As @Olivier has correctly answered the y coordinate is the baseline.

But, as the $angle is 0 and the bounding-box at hand, we only need to correct by an upper y value, e.g. offsets 5 or 7:

        Corners of GD Bounding Boxes
                                               x,y
     (4)<--(3)       (1) lower-left corner     0,1
      :     ^        (2) lower-right corner    2,3
      .     |        (3) upper-right corner    4,5
     (1)-->(2)       (4) upper-left corner     6,7

The value is negative by the number of pixels above the baseline.

$y = $centerY - $top - $top_offset;

It can make sense to also do an $offsetX correction (upper-left.x, offset 6), but it is often minimal only:

$x = $centerX - $offsetX + intdiv($offsetX, 2) - $left_offset;

Images with these two corrections (X+Y), and the projected bounding box from the imageftbbox() function as a red rectangle on top:

Image	Image	Image	Image