there is no such thing as a "compressed bitmap". Bitmaps are inherently
uncompressed. Therefore, when you are reading the tiff image into a bitmap
that you created, eg...

dim btmap as new bitmap("yourfile.tiff")
you are actually creating a bitmap with 4 channels ARGB.

You can't compress the btmp in memory. You can, however, save the channels
into an array, and if you ignore the A channel, you can reduce the memory
footprint.

If you want to compress the output file, you can save the bitmap as jpg,
like btmap.save()