Question

I am trying to create a transparent image canvas and then place other random images on top of this canvas. I then finally save the final image as a gif. I have tried the below:

    $canvas = imagecreatetruecolor($this->canvas_width, $this->canvas_height);
    imagesavealpha($canvas, true);
    imagealphablending($canvas, false);       

    $trans_colour = imagecolorallocatealpha($canvas, 0, 0, 0, 127);
    imagefill($canvas, 0, 0, $trans_colour); 

However, if there is some unused space left on the canvas then this part is black. I thought this area should be transparent?

Am I applying transparency correctly with the above?

Answer 1

imagesavealpha can not be used for images you eventually intend to save as GIF because GIF images do not have alpha channels. From the docs:

imagesavealpha — Set the flag to save full alpha channel
information (as opposed to single-color transparency) when saving PNG
images

So it only works if you are going to save the image as PNG. GIF images can do transparency, but not with an alpha channel; they can only be “transparent” or “not transparent” by defining a particular color as transparent. In PHP you can do that using imagecolortransparent. For example, to make all black transparent, you could do:

$black_color = imagecolorallocate($canvas, 0, 0, 0);
imagecolortransparent($canvas, $black_color);