If an image is filled with a solid color (rgb 0,0,0) the average color will be 0,0,0, and the histogram will have the maximum values at r[0], g[0], and b[0]. Since we remove those values with the tolerance (+- 16), we are only left with colors outside of average.
If the removed colors represent more than 90% of the image (we remove 6.25-12.5% of the histogram based on the average color), then we asume the image is blank.
This detects solid filled images of any color, and with images of very little color variance (blank wall, blue sky with no clouds, nearly faded images, very low contrast and low color range images). The function is perfect for checking if a frame from a video is blank or not (in case you are generating thumbnails).
<?php
function isBlank($gd_resource, $tolerance, $percent)
{
list($w,$h) = array(imagesx($gd_resource), imagesy($gd_resource));
$count = 0;
$hr = $hg = $hb = array();
for ($i=0; $i<$h; $i++) {
for ($j=0; $j<$w; $j++) {
$pos = imagecolorat($gd_resource, $j, $i);
$f = imagecolorsforindex($gd_resource, $pos);
$hr[$f['red']] = issetor($hr[$f['red']],0) + 1;
$hg[$f['green']] = issetor($hg[$f['green']],0) + 1;
$hb[$f['blue']] = issetor($hb[$f['blue']],0) + 1;
$count ++;
}
}
$rc = array_sum($hr);
$gc = array_sum($hg);
$bc = array_sum($hb);
$r = $rc/$count;
$g = $gc/$count;
$b = $bc/$count;
$ra = range(max(0,$r-$tolerance), min(255,$r+$tolerance));
$ga = range(max(0,$g-$tolerance), min(255,$g+$tolerance));
$ba = range(max(0,$b-$tolerance), min(255,$b+$tolerance));
foreach ($ra as $rx) {
unset($hr[$rx]);
}
foreach ($ga as $gx) {
unset($hg[$gx]);
}
foreach ($ba as $bx) {
unset($hb[$bx]);
}
$red = floatval(array_sum($hr)+1) / floatval($rc);
$green = floatval(array_sum($hg)+1) / floatval($gc);
$blue = floatval(array_sum($hb)+1) / floatval($bc);
if ($red<$percent && $green<$percent && $blue<$percent) {
return true;
}
return false;
}
isBlank($gd_resource, 16, 0.1);
?>
