Re: Creating PNG file with transparency

On Wed, 8 Oct 2008, Nguyen Vu Hung wrote:

> Hello all,

> In google map[1] , the satellite image and the "road" image[2] are 
> separated.The browser then look at the "Opacity" of the "road" 
> image[2] and makeit transparent.

> My questions are:

> 1. How to use GM's Image class[0] to make a transparent image like 
> google.png2. What does "Opacity: ( 0, 0, 0,255)  #000000FF" means in 
> the output of gm identify -verbose -verbose google.trans.png.  What 
> API in GM's Image class do I need?3. Do I need to set "Matte Color: 
> grey74" and "Background Color:grey100"[3] if I want to do 1. ?

While most graphics programs use an alpha channel which represents the 
degree of transparency, GraphicsMagich has an opacity channel which 
represents the degree of opacity (inverse of transparency).  An opaque 
image has an opacity value of 0 and a completely transparent pixel has 
an opacity value of MaxRGB.  In the "0, 0, 0,255", the 255 indicates 
completely transparent.  Similarly, the "FF" is max range in the color 
specification and indicates completely transparent.

PNG and GIF have the ability to assign a single color as being the 
"transparent".  This requires a colormapped image representation. 
GraphicsMagick has a special "PNG8" output option to support this 
binary transparency.

> A sample code would be nice if you are kind enough.

While I will not write the entire program for you, causing an image 
color to be transparent is quite easy to do via the 'transparent' 
method of the Image class.  For example

   image.transparent("white");

will set any white color in the image to transparent.  You can 
explicity write PNG8 format like

   image.write("PNG8:output.png").

You can also create a transparent white canvas image and draw or 
composite on top of it, but this may obtain full alpha transparency 
rather than simple binary transparency.

Bob
======================================
Bob Friesenhahn
[email protected], http://www.simplesystems.org/users/bfriesen/
GraphicsMagick Maintainer,    http://www.GraphicsMagick.org/


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