PHP gdLib 8-Bit PNG with alpha

Question

how is it possible to save my image, created with gd, as an png-8?

it saves as gif with transparent channel well - but I want to use png-8.

Best Regards, Beerweasle

Answer 1

Using imagesavealpha() and a transparent bg color should do the trick...

Based on dfilkovi's code:

<?php
// Create a new true color image
$im = new imagecreatetruecolor(100, 100);

// Fill with alpha background
$alphabg = imagecolorallocatealpha($im, 0, 0, 0, 127);
imagefill($im, 0, 0, $alphabg);

// Convert to palette-based with no dithering and 255 colors with alpha
imagetruecolortopalette($im, false, 255);
imagesavealpha($im, true);

// Save the image
imagepng($im, './paletteimage.png');
imagedestroy($im);
?>

Answer 2

@Sonny

false assumption: PNG of any bit depth can have transparency. It is recorded in the tRNS chunk of the png image (except for truecolor ones) cf format definition

cf www.libpng.org/pub/png/spec/1.2/PNG-Chunks.html#C.tRNS

idem www.w3.org/TR/PNG-Chunks.html#C.tRNS

The difference is how it is recorder: RGBA has a unique record per pixel, with 4 values (3 colors and 1 alpha channel), where "paletted" PNG records alpha channel in its own chunk.

Fireworks is very good at it.

Examples:

http://www.libpng.org/pub/png/pngs-img.html