Image as isosceles trapezoid PHP GD - php

Question

i have this function that transforms image to trapezoid using PHP GD:
function perspective($i,$gradient=0.9,$rightdown=true,$background=0xFFFFFF) {
$mult=3;
$w=imagesx($i);
$h=imagesy($i);
$image=imagecreatetruecolor($w*$mult,$h*$mult);
imagecopyresized($image,$i,0,0,0,0,$w*$mult,$h*$mult,$w,$h);
imagedestroy($i);
$w*=$mult;
$h*=$mult;
$im=imagecreatetruecolor($w,$h);
$background=imagecolorallocate($im,($background>>16)&0xFF,($background>>8)&0xFF,$background&0xFF);
imagefill($im,0,0,$background);
imageantialias($im,true);
$nh=$h-($h*$gradient);
for ($x=0; $x<$w; $x++) {
$ni=(($rightdown) ? $x : $w-$x);
$p=intval($h-(($ni/$w)*$nh));
if (($p%2)<>0)
$p-=1;
$nx=intval(($p-$h)/2);
imagecopyresampled($im,$image,$x,0,$x,$nx,1,$p,1,$h-1);
imageline($im,$x,$h-1,$x,$h+$nx,$background);
imageline($im,$x,0,$x,-$nx-1,$background);
}
imagedestroy($image);
imagefilter($im,IMG_FILTER_SMOOTH,10);
$i=imagecreatetruecolor($w/$mult,$h/$mult);
imageantialias($i,true);
imagecopyresampled($i,$im,0,0,0,0,$w,$h,$w*$mult,$h*$mult);
imagedestroy($im);
return $i;
}
But i cant modify it to produce isosceles trapezoid, i think there needs just one small modification, but i cant figure it outh (i tried lot of things).
Can someone help?

Answer 1

Right, basically that code should generate the right values, but due to a bug has a lot of cludges in place to get a trapezium shape. The bug is that the copy of each line has the destination-y and the source-y values transposed. The source-y should always be 0, the destination-y should change.
There were also a few other small numerical bugs and double rounding at unnecessary points throwing off the results.
Also, the variable naming was atrocious, so I have rewritten it so that the entire function is clear.
Try the following:
function makeTrapeziumImage($image, $gradient, $rightToLeft = false, $background = 0xFFFFFF, $supersampleScale = 3) {
$originalWidth = imagesx($image);
$originalHeight = imagesy($image);
$supersampledWidth = $originalWidth * $supersampleScale;
$supersampledHeight = $originalHeight * $supersampleScale;
$supersampledImage = imagecreatetruecolor($supersampledWidth, $supersampledHeight);
imagecopyresized($supersampledImage, $image,
0, 0, 0, 0,
$supersampledWidth, $supersampledHeight, $originalWidth, $originalHeight);
$workingImage = imagecreatetruecolor($supersampledWidth, $supersampledHeight);
$backgroundColour = imagecolorallocate($workingImage, ($background >> 16) & 0xFF, ($background >> 8) & 0xFF, $background & 0xFF);
imagefill($workingImage, 0, 0, $backgroundColour);
imageantialias($workingImage,true);
$endHeight = $supersampledHeight - ($supersampledHeight * $gradient);
for ($x = 0; $x < $supersampledWidth; $x++) {
$cX = ($rightToLeft ? $supersampledWidth - $x : $x);
$dstHeight = $supersampledHeight - ((($cX + 1) / $supersampledWidth) * $endHeight);
$dstY = intval(($supersampledHeight - $dstHeight) / 2) - 1; // -1 required as zero-indexed
$dstY = ($dstY < 0 ? 0 : $dstY); // Rounding can make $dstY = -1
$dstHeight = intval($dstHeight); // Round the height after calculating $dstY
imagecopyresampled($workingImage, $supersampledImage,
$cX, $dstY, $cX, 0,
1, $dstHeight, 1, $supersampledHeight);
}
imagedestroy($supersampledImage);
imagefilter($workingImage, IMG_FILTER_SMOOTH, 10);
$resizedImage = imagecreatetruecolor($originalWidth, $originalHeight);
imageantialias($resizedImage, true);
imagecopyresampled($resizedImage, $workingImage,
0, 0, 0, 0,
$originalWidth, $originalHeight, $supersampledWidth, $supersampledHeight);
imagedestroy($workingImage);
return $resizedImage;
}
The essential mechanism of the inner loop, as before, is to take each column of pixels, along the x-axis and resize them over a gradient. It naturally creates an isosceles trapezium. In order to create a non-isosceles trapezoid, another gradient would have to be specified. Alternatively, a set of start and end y-values could be specified and the gradients calculated from them.
Whilst this example works along the x-axis, in either direction as before, it could just as easily work along the y-axis (or the image could be rotated 90 degrees, processed, then rotated back).