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Pure python PNG writer
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import numpy as np | |
import zlib | |
import struct | |
def make_png(data): | |
""" | |
Convert numpy array to PNG byte array. | |
*data* must be (H, W, 4) with dtype=ubyte | |
""" | |
# www.libpng.org/pub/png/spec/1.2/PNG-Structure.html | |
header = b'\x89PNG\x0d\x0a\x1a\x0a' # header | |
def mkchunk(data, name): | |
if isinstance(data, np.ndarray): | |
size = data.nbytes | |
else: | |
size = len(data) | |
chunk = np.empty(size + 12, dtype=np.ubyte) | |
chunk.data[0:4] = struct.pack('!I', size) | |
chunk.data[4:8] = name # b'CPXS' # critical, public, standard, safe | |
chunk.data[8:8+size] = data | |
chunk.data[-4:] = struct.pack('!i', zlib.crc32(chunk[4:-4])) | |
return chunk | |
# www.libpng.org/pub/png/spec/1.2/PNG-Chunks.html#C.IHDR | |
ctyp = 0b0110 # alpha, color | |
h, w = data.shape[:2] | |
depth = data.itemsize * 8 | |
ihdr = struct.pack('!IIBBBBB', w, h, depth, ctyp, 0, 0, 0) | |
c1 = mkchunk(ihdr, 'IHDR') | |
# www.libpng.org/pub/png/spec/1.2/PNG-Chunks.html#C.IDAT | |
idat = np.empty((h, w*4 + 1), dtype=np.ubyte) # insert filter byte at each scanline | |
idat[:, 1:] = data.reshape(h, w*4) | |
idat[:, 0] = 0 | |
c2 = mkchunk(zlib.compress(idat), 'IDAT') | |
c3 = mkchunk(np.empty((0,), dtype=np.ubyte), 'IEND') | |
# concatenate | |
lh = len(header) | |
png = np.empty(lh + c1.nbytes + c2.nbytes + c3.nbytes, dtype=np.ubyte) | |
png.data[:lh] = header | |
p = lh | |
for chunk in (c1, c2, c3): | |
png[p:p+len(chunk)] = chunk | |
p += chunk.nbytes | |
return png | |
if __name__ == '__main__': | |
data = np.zeros((100,100,4), dtype=np.ubyte) | |
data[:,:,3] = 255 | |
data[:,:,0] = 100 | |
data[10,:] = (0,0,0,255) | |
data[:,10] = (0,0,0,255) | |
open('test.png', 'w').write(make_png(data)) | |
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