Created
June 15, 2010 12:51
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def nearly_eq(value,target_value,off_by): | |
""" | |
an equality function with a margin for error (off_by) | |
""" | |
return target_value-off_by < value < target_value+off_by | |
def make_transparent(blob, threshold=210, tolerance=30): | |
r = png.Reader(bytes=blob) | |
(width,height,pixels,meta) = r.asRGBA8() | |
w = png.Writer( | |
width=width, | |
height=height, | |
alpha=True, | |
compression=None | |
) | |
output = StringIO.StringIO() | |
# do we need tollerence? | |
use_tolerance = True | |
if threshold+tolerance >= 255: | |
use_tolerance = False | |
# process | |
new_pixels = [] | |
for row in pixels: | |
new_row = array.array('B', row) | |
i = 0 | |
while i < len(row): | |
if row[i]>threshold and row[i+1]>threshold and row[i+2]>threshold: | |
if use_tolerance: | |
m = int((row[i]+row[i+1]+row[i+2])/3.0) | |
if nearly_eq(row[i],m,tolerance) and nearly_eq(row[i+1],m,tolerance) and nearly_eq(row[i+2],m,tolerance): | |
new_row[i + 3] = 0 | |
else: | |
new_row[i + 3] = 0 | |
i += 4 | |
new_pixels.append(new_row) | |
# store | |
w.write_passes(output,new_pixels,packed=False) | |
blob = db.Blob(output.getvalue()) | |
output.close() | |
return blob |
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