I am using PHP to test out my bilinear algorithm. The code is un-optimized for clarity.
Here is what the code below is doing:
Note: The images here are resized from 10x10 to 100x100 for better viewing.
However, it does not match the result I get in Photoshop using bilinear resampling:
Full source code:
<?php
$image1 = imagecreatefrompng( 'test.png' );
$w1 = imagesx( $image1 );
$h1 = imagesy( $image1 );
$w2 = 10;
$h2 = 10;
$image2 = imagecreatetruecolor( $w2, $h2 );
imagefill($image2, 0, 0, imagecolorallocate($image2, 0x4c, 0x4c, 0x8e)); // added bg for pixels to stand-out
function lerp($v0, $v1, $t) {
return $v0 + $t*($v1-$v0);
}
function getPixel($image, $x, $y){
$rgb = imagecolorat( $image, $x, $y );
$r = ($rgb >> 16) & 0xFF;
$g = ($rgb >> 8) & 0xFF;
$b = $rgb & 0xFF;
return array($r,$g,$b);
}
$maxY1 = $h1 - 1;
$maxX1 = $w1 - 1;
$maxY2 = $h2 - 1;
$maxX2 = $w2 - 1;
// plot original pixels from source to destination
for($y = 0; $y <= $maxY1; $y++) { // loop thru src height
$newY = floor(($y/$maxY1) * $maxY2);
for ($x = 0; $x <= $maxX1; $x++) { // loop thru src width
$newX = floor(($x/$maxX1) * $maxX2);
$rgb = imagecolorat( $image1, $x, $y );
$r1 = ($rgb >> 16) & 0xFF;
$g1 = ($rgb >> 8) & 0xFF;
$b1 = $rgb & 0xFF;
imagesetpixel( $image2, $newX, $newY, imagecolorallocate( $image2, $r1, $g1, $b1 ) );
}
}
imagepng( $image2, 'out1.png' );
// interpolate pixels from pixel[1,0] to pixel[8,0]
$y = 0;
$rgb = imagecolorat( $image2, 0, $y );
$r0 = ($rgb >> 16) & 0xFF;
$g0 = ($rgb >> 8) & 0xFF;
$b0 = $rgb & 0xFF;
$rgb = imagecolorat( $image2, 9, $y );
$r1 = ($rgb >> 16) & 0xFF;
$g1 = ($rgb >> 8) & 0xFF;
$b1 = $rgb & 0xFF;
for($x=1; $x <= 8; $x++){
$t = $x / 9;
$r = lerp($r0, $r1, $t);
$g = lerp($g0, $g1, $t);
$b = lerp($b0, $b1, $t);
imagesetpixel( $image2, $x, $y, imagecolorallocate( $image2, $r, $g, $b ) );
}
imagepng( $image2, 'out2.png' );
// interpolate pixels from pixel[1,9] to pixel[8,9]
$y = 9;
$rgb = imagecolorat( $image2, 0, $y );
$r0 = ($rgb >> 16) & 0xFF;
$g0 = ($rgb >> 8) & 0xFF;
$b0 = $rgb & 0xFF;
$rgb = imagecolorat( $image2, 9, $y );
$r1 = ($rgb >> 16) & 0xFF;
$g1 = ($rgb >> 8) & 0xFF;
$b1 = $rgb & 0xFF;
for($x=1; $x <= 8; $x++){
$t = $x / 9;
$r = lerp($r0, $r1, $t);
$g = lerp($g0, $g1, $t);
$b = lerp($b0, $b1, $t);
imagesetpixel( $image2, $x, $y, imagecolorallocate( $image2, $r, $g, $b ) );
}
imagepng( $image2, 'out3.png' );
// interpolate remaining pixels
for($x=0; $x <= 9; $x++){
$rgb = imagecolorat( $image2, $x, 0 );
$r0 = ($rgb >> 16) & 0xFF;
$g0 = ($rgb >> 8) & 0xFF;
$b0 = $rgb & 0xFF;
$rgb = imagecolorat( $image2, $x, 9 );
$r1 = ($rgb >> 16) & 0xFF;
$g1 = ($rgb >> 8) & 0xFF;
$b1 = $rgb & 0xFF;
for($y = 1; $y <= 8; $y++){
$t = $y / 9;
$r = lerp($r0, $r1, $t);
$g = lerp($g0, $g1, $t);
$b = lerp($b0, $b1, $t);
imagesetpixel( $image2, $x, $y, imagecolorallocate( $image2, $r, $g, $b ) );
}
}
imagepng( $image2, 'out4.png' );
header('Content-type: image/png');
imagepng( $image2);
imagedestroy( $image1 );
What am I missing?
Photoshop is correct. In your version the original 4 pixel values end up in the extreme corners of the new image but in the correct bilinear interpolation they end up in the centers of 4 quadrants of the new image. There is no information beyond the edge of the original image so photoshop does constant extrapolation at the edge:
2x2:
10x10 before interpolation:
If you started with a 3x3 image instead of 2x2, your method would cause the original edge pixels to have a diminished contribution to the final image relative to the center pixels, biasing the result.