How to specify a custom channel distribution in ImageMagick? - image

Question

I've got a bunch of files that are not e. g. RGB16 but essentially have a custom format.
This format has 16 bits, and the channels are divided as follows, and I want ImageMagick to import them as such and then export them to a readable, regular RGBA format (or alternatively, export each channel into a seperate grayscale file).
Bit 0-4: Red
Bit 5-9: Green
Bit 10-12: Blue
Bit 13-15: Alpha
So essentially 5-5-3-3.
Here is a zipped sample file, the wanted output (in PNG) is also included.
Does anyone know how to do that?
I succeeded in doing this in a different, manual way, but I have no idea how to do this in ImageMagick.

Answer 1

The first thing to do is take the expected output you kindly provided and separate that into its constituent RGBA channels and lay them out side-by-side so we can see what we are aiming for in each channel:
magick out.png -separate +smush 10 channels.png
Next thing is to check the file size. You said 512x512 pixels of 16-bit data. That means I am expecting 512x512x2, i.e. 524,288 bytes but your file has 524,290 bytes. A quick examination shows you have a 2-byte BOM at the start that we need to skip over. That means our command must treat the data as 512x512 pixels of 16-bit greyscale data with a 2-byte header to skip. We also do not want to read the file 4 times, so we put it into an MPR "Magick Persistent Register" (or "lump of named memory" is how I think of it) and recall the original data from there each time as we decode the channels.
So, in the command below, the first line reads the input image, skipping the BOM and puts it into the MPR. The second line takes a copy of the MPR and bit-twiddles the Red channel out of it. The second, third and fourth lines do the same for the Green, Blue and Alpha channels. The final line combines the 4 channels in the processing stack into the output file.
magick -size 512x512+2 -endian MSB -depth 16 gray:in.file -write MPR:orig +delete \
\( MPR:orig -evaluate and 63488 \) \
\( MPR:orig -evaluate and 1984 -evaluate leftshift 5 \) \
\( MPR:orig -evaluate and 56 -evaluate leftshift 10 \) \
\( MPR:orig -evaluate and 7 -evaluate leftshift 13 \) \
-combine result.png
Note:
63488 = 0xf800
1984 = 0x07c0
56 = 0x0038
On Windows, which I don't use and cannot test, that command will look something like this:
magick -size 512x512+2 -endian MSB -depth 16 gray:in.file -write MPR:orig +delete ^
( MPR:orig -evaluate and 63488 ) ^
( MPR:orig -evaluate and 1984 -evaluate leftshift 5 ) ^
( MPR:orig -evaluate and 56 -evaluate leftshift 10 ) ^
( MPR:orig -evaluate and 7 -evaluate leftshift 13 ) ^
-combine result.png
Just for fun, you can do the same thing in Python:
#!/usr/bin/env python3
import numpy as np
from PIL import Image
h, w = 512, 512
# Read raw 16-bit file, skipping BOM with offset=2. Reshape.
raw = np.fromfile('in.file', dtype=np.dtype('>u2'), offset=2).reshape((h,w))
# RGBA5533 packed into uint16
R = (np.bitwise_and(raw, 0xf800) >> 8).astype(np.uint8)
G = (np.bitwise_and(raw, 0x07c0) >> 3).astype(np.uint8)
B = (np.bitwise_and(raw, 0x0038) << 2).astype(np.uint8)
A = (np.bitwise_and(raw, 0x0007) << 5).astype(np.uint8)
# Stack the individual channels into RGBA
RGBA = np.dstack((R,G,B,A))
# Display or save
Image.fromarray(RGBA).show()