TiffImagePlugin doesn't honor the ROWSPERSTRIP tag value

[agiudiceandrea]

What did you do?

I tried to save a PIL.Image.Image (mode=RGBA, size=335x333, 8 bits per sample) to a TIFF file with LZW compression and 9 rows per strip.

img.save('output.tiff', compression='tiff_lzw', tiffinfo={278: 9})

What did you expect to happen?

The file to be saved as a TIFF file with LZW compression and 9 rows per strip.

What actually happened?

The file was saved as a TIFF file with LZW compression and 48 rows per strip.

What are your OS, Python and Pillow versions?

• OS: Windows 10 64-bit
• Python: 3.9.5
• Pillow: 9.2.0

[radarhere]

As context, ROWSPERSTRIP has been set by TiffImagePlugin since PIL.

If I make a change to allow ROWSPERSTRIP to be set by the user, the following test code starts failing.

Pillow/Tests/test_file_tiff_metadata.py

Lines 124 to 128 in 53b6e5f

	def test_write_metadata(tmp_path):
	"""Test metadata writing through the python code"""
	with Image.open("Tests/images/hopper.tif") as img:
	f = str(tmp_path / "temp.tiff")
	img.save(f, tiffinfo=img.tag)

The error is

Pillow/src/PIL/TiffImagePlugin.py

Line 906 in 53b6e5f

raise NotImplementedError("multistrip support not yet implemented")

[agiudiceandrea]

@radarhere, thanks.
Opening an image with a certain number of rows per strip and just saving it without any manipulation and with the same tags:

sourceimg = Image.open('input.tiff')
tiffinfo = sourceimg.tag_v2
sourceimg.save('output', tiffinfo=tiffinfo)

then the saved image will have a different number of rows per strip.

Anyway, changing the TiffImagePlugin.ROWSPERSTRIP tag number to another bogus non conflicting tag number, e.g. 10, it is possible to trick the TiffImagePlugin, so it will write its calculated value of the rows per strip to such bogus tag number 10 so the tiff library will use the rows per strip provided by the tiffinfo:

sourceimg = Image.open('input.tiff')
tiffinfo = sourceimg.tag_v2
TiffImagePlugin.ROWSPERSTRIP = 10
sourceimg.save('output', tiffinfo=tiffinfo)

However, this is not a complete workaround, as the saved image will have a spurious tag number 10.

[radarhere]

Changing TiffImagePlugin.ROWSPERSTRIP is an inventive idea, but I think it might break reading TIFF files afterwards.

If you're looking for a workaround, see what you think of this.

from PIL import Image, TiffImagePlugin

def setRowsPerStrip(im, rows_per_strip):
    TiffImagePlugin.WRITE_LIBTIFF = True
    
    bits = TiffImagePlugin.SAVE_INFO[im.mode][4]
    stride = len(bits) * ((im.size[0] * bits[0] + 7) // 8)
    TiffImagePlugin.STRIP_SIZE = rows_per_strip * stride

im = Image.open("Tests/images/hopper.tif")
setRowsPerStrip(im, 110)
im.save("out.tif")

[radarhere]

I've created #7654 to resolve this.