tiffsave xres/yres arguments don't match docstring

[f--f]

I noticed that the docstring for tiffsave describes the optional xres and yres arguments like this:

xres (float): Horizontal resolution in pixels/mm
yres (float): Vertical resolution in pixels/mm

But running

pyvips.Image.grey(100, 100).tiffsave("grey.tif", xres=100, yres=100)

and then checking the result with tiffinfo outputs Resolution: 100, 100 pixels/cm (xres and yres in pixels/cm, and not pixels/mm - this is the case even if resunit is specified as inches).

I suspect the confusion is that libvips stores xres/yres metadata in pixels/mm, which does get converted properly if it is specified in tiffsave.

The relevant section of existing code for handling resolution units seems to be here:

libvips/libvips/foreign/tiffsave.c

Lines 145 to 165 in 2da56b9

	/* Default xres/yres to the values from the image.
	*/
	if( !vips_object_argument_isset( object, "xres" ) )
	tiff->xres = save->ready->Xres * 10.0;
	if( !vips_object_argument_isset( object, "yres" ) )
	tiff->yres = save->ready->Yres * 10.0;
	/* resunit param overrides resunit metadata.
	*/
	if( !vips_object_argument_isset( object, "resunit" ) &&
	vips_image_get_typeof( save->ready,
	VIPS_META_RESOLUTION_UNIT ) &&
	!vips_image_get_string( save->ready,
	VIPS_META_RESOLUTION_UNIT, &p ) &&
	vips_isprefix( "in", p ) )
	tiff->resunit = VIPS_FOREIGN_TIFF_RESUNIT_INCH;
	if( tiff->resunit == VIPS_FOREIGN_TIFF_RESUNIT_INCH ) {
	tiff->xres *= 2.54;
	tiff->yres *= 2.54;
	}

(Using vips 8.8.0)

[jcupitt]

Ooop, you're right, it's all messed up. I'll have a look. Thanks!

[jcupitt]

I think it's working now. There was a *10 missing from one side of an if :-( Thanks again for reporting this, it'll be in 8.8.2.

$ vips copy k2.jpg x.tif[strip]
$ vips copy x.tif y.tif[xres=100]
$ tiffinfo y.tif
TIFF Directory at offset 0x87f008 (8908808)
  Image Width: 1450 Image Length: 2048
  Resolution: 1000, 28.3465 pixels/cm
  Bits/Sample: 8
  Sample Format: unsigned integer
  Compression Scheme: None
  Photometric Interpretation: RGB color
  Orientation: row 0 top, col 0 lhs
  Samples/Pixel: 3
  Rows/Strip: 128
  Planar Configuration: single image plane
$ vips copy x.tif y.tif[xres=100,resunit=inch]
$ tiffinfo y.tif
TIFF Directory at offset 0x87f008 (8908808)
  Image Width: 1450 Image Length: 2048
  Resolution: 2540, 72 pixels/inch
  Bits/Sample: 8
  Sample Format: unsigned integer
  Compression Scheme: None
  Photometric Interpretation: RGB color
  Orientation: row 0 top, col 0 lhs
  Samples/Pixel: 3
  Rows/Strip: 128
  Planar Configuration: single image plane

[f--f]

😁 Looks good, thanks!

[jcupitt]

I added some tests too:

f80c7a1