Region.fetch operator yields unexpected buffer size when operating on a .v image

[erdmann]

Hi John,

I'm experimenting with the new Region.fetch functionality, and I encountered an unexpected result. When I operate from a pipeline, it behaves as expected, but when operating from a .v file opened directly, it gives nonsensical buffer lengths:

import pyvips # version 2.1.6

b = pyvips.Image.black(1000, 1000)
reg = pyvips.Region.new(b)
print('buffer len from pipeline: ', len(reg.fetch(0, 0, 100, 100)))
b.write_to_file('black.v')


v_file = pyvips.Image.new_from_file('black.v')
reg2 = pyvips.Region.new(v_file)
print('buffer len from v file: ', len(reg2.fetch(0, 0, 100, 100)))

print('v_file:', v_file)

reg3 = pyvips.Region.new(v_file.cast('uchar'))
print('buffer len from v file in a do-nothing pipeline: ', len(reg3.fetch(0, 0, 100, 100)))

The result is as follows:

buffer len from pipeline:  10000 # expected
buffer len from v file:  356000 # unexpected
v_file: <pyvips.Image 1000x1000 uchar, 1 bands, multiband>
buffer len from v file in a do-nothing pipeline:  10000 # expected

Many thanks in advance for taking a look.

[jcupitt]

Oh hmm yes you're right, it won't work properly in cases like that.

I'll have a think.

[jcupitt]

OK, 8.8.1 should have this fix. Thanks!

[erdmann]

Works for me now. Thanks again!