How can I save a bmp (bitmap) image file at 16bpp on a Mac?

Question

I need to save an image as Windows Bitmap BMP A1R5G5B5 16bpp.
Source files are 32bpp image files (PNG and/or Windows Bitmap).

Preview saves BMP images at 32bpp.

XnConvert saves BMP images at 24bpp.

Imagemagick, Acorn, GraphicConverter do not support saving at 16bpp.

Photoshop can do it, but I don't own it.

Any ideas? GUI or command line is fine for me.

Answer 1

This rather quick – and possibly dirty – Python program will read a 32bpp BMP image, assert it is indeed 32 bpp, and if so convert it to a 16-bit ARGB image and write out to a new file.

import sys, struct

def convertBmp(filename):
    with open(filename, 'rb') as in_file:
        data = in_file.read()

    type1,type2,size,reserved1,reserved2,offset,dib,wide,high,planes,bpp = struct.unpack('<ccIHHIIIIHH',data[:30])
    if type1 != b'B' or type2 != b'M':
        raise ValueError('Not a BMP image')
    if planes != 1 or bpp != 32:
        raise ValueError('Not a 32bpp BMP image')

    header = bytearray(data[:offset])
    data = bytearray(data[offset:])
    # process data
    newdata = bytearray()
    for y in range(high):
        for x in range(wide):
            b,g,r,a = data[4*wide*y+4*x:4*wide*y+4*x+4]
            a = 32768 if a < 127 else 0
            r >>= 3
            g >>= 3
            b >>= 3
            newdata += struct.pack('<H',a+(r<<10)+(g<<5)+b)
        # add row padding
        if wide & 1:
            newdata += b'\x00\x00'
    # update total size
    header[2:6] = struct.pack('<I',len(header)+len(newdata))
    # update bpp
    header[28] = 16
    # update planes
    header[30] = 0
    # update raw image size
    header[34:38] = struct.pack('<I',len(newdata))
    return header+newdata

if len(sys.argv) != 3:
    print ('usage: python convbmp.py input.bmp output.bmp')
else:
    image = convertBmp(sys.argv[1])
    with open(sys.argv[1], 'wb') as out_file:
        out_file.write (image)

It should only touch the BMP data fields that are actually changed for the conversion and leave the rest alone. Testing was a bit troublesome as I don't appear to have any software to view the resulting ARGB; both Photoshop and Apple's Preview (again!) let me down by not showing if this one-bit alpha channel actually worked. Photoshop always seems to import the alpha channel as if everything is set. (If alpha does not work for you, experiment with the value of the setting of that a variable; its logic may need inverting.)

This should cover the basics. Expanding it to be able to accept multiple arguments or loop over an entire folder of data, is left as an implementation detail.