imagealphablending() allows for
two different modes of drawing on truecolor images. In
blending mode, the alpha channel component of the color
supplied to all drawing function, such as
imagesetpixel() determines how much of the underlying
color should be allowed to shine through. As a result, gd
automatically blends the existing color at that point with
the drawing color, and stores the result in the image. The
resulting pixel is opaque. In non-blending mode, the drawing
color is copied literally with its alpha channel information,
replacing the destination pixel. Blending mode is not
available when drawing on palette images. If blendmode is
TRUE, then blending mode is enabled, otherwise
disabled.
注: This function was added in PHP 4.0.6 and requires GD 2.0.1