[Image-SIG] Re: how to make a transparent background

[Rob Myroon]

Hi Victor,

I think I solved my problem.  Remember ... I want to
save .png files with a transparent color.  I have
modified the PngImagePlugin.py file.  There already is
code to set a transparent color but it doesn't seem to
get executed.  My change executes that code.

To set a transparent color simply add a "transparency"
entry to the image's info dictionary.  (Don't do this
if you load a transparent gif or a transparent png ---
PIL creates a "transparency" entry in the info
dictionary.)

The image file must be "P" (palette).  ("L" might work
too but I haven't tried it.)

ex.

im = Image.open(file.xxx)    # ex. 8-bit .bmp
im.info["transparency"] = color number in the palette 
im.save("newfile.png")


Here is a section of code and my additions from
PngImagePlugin.py

def _save(im, fp, filename, chunk=putchunk, check=0):
    # save an image to disk (called by the save
method)

    .
    .
    .

    chunk(fp, "IHDR",
          o32(im.size[0]), o32(im.size[1]),     #  0:
size
          mode,                                 #  8:
depth/type
          chr(0),                               # 10:
compression
          chr(0),                               # 11:
filter category
          chr(0))                               # 12:
interlace flag

    if im.mode == "P":
        chunk(fp, "PLTE", im.im.getpalette("RGB"))

    # Added by R. Myroon 2002.11.06
    if not im.encoderinfo.has_key("transparency"):
        if im.info.has_key("transparency"):
            im.encoderinfo["transparency"] =
im.info["transparency"]
    # End code 2002.11.06
    
    if im.encoderinfo.has_key("transparency"):
        if im.mode == "P":
            transparency = max(0, min(255,
im.encoderinfo["transparency"]))
            chunk(fp, "tRNS", chr(255) * transparency
+ chr(0))
        elif im.mode == "L":
            transparency = max(0, min(65535,
im.encoderinfo["transparency"]))
            chunk(fp, "tRNS", o16(transparency))
        else:
            raise IOError, "cannot use transparency
for this mode"


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