Saving a greyscale tiff pyramid fails to add predictor tags at lower levels about libvips

[kleisauke]

IIUC, I think the first part of the patch mentioned in comment #3808 (comment) is still needed.

@@ -684,7 +684,7 @@ wtiff_compress_jpeg_header(Wtiff *wtiff,
        jpeg_set_defaults(cinfo);

        // use RGB mode (no chroma subsample) for high Q
-       if (wtiff->Q >= 90)
+       if (wtiff->Q >= 90 && image->Bands == 3)
                jpeg_set_colorspace(cinfo, JCS_RGB);

        /* Set compression quality. Must be called after setting params above.

(perhaps wtiff->Q >= 90 && space == JCS_RGB is clearer?)

[manthey]

A fix might be this diff

diff --git a/libvips/foreign/vips2tiff.c b/libvips/foreign/vips2tiff.c
index 3d8ccc06e..b6ad200b8 100644
--- a/libvips/foreign/vips2tiff.c
+++ b/libvips/foreign/vips2tiff.c
@@ -684,7 +684,7 @@ wtiff_compress_jpeg_header(Wtiff *wtiff,
        jpeg_set_defaults(cinfo);

        // use RGB mode (no chroma subsample) for high Q
-       if (wtiff->Q >= 90)
+       if (wtiff->Q >= 90 && image->Bands == 3)
                jpeg_set_colorspace(cinfo, JCS_RGB);

        /* Set compression quality. Must be called after setting params above.
@@ -2335,6 +2335,11 @@ wtiff_copy_tiff(Wtiff *wtiff, TIFF *out, TIFF *in)
         * we copy raw tiles.
         */

+       if ((wtiff->compression == COMPRESSION_ADOBE_DEFLATE ||
+                       wtiff->compression == COMPRESSION_LZW) &&
+               wtiff->predictor != VIPS_FOREIGN_TIFF_PREDICTOR_NONE)
+               TIFFSetField(out, TIFFTAG_PREDICTOR, wtiff->predictor);
+
        /* We can't copy profiles or xmp :( Set again from wtiff.
         */
        if (wtiff_embed_xmp(wtiff, out) ||

But I'm not understanding why writing the prediction tag was removed for the webp and zstd compression formats, so don't know if that should be changed back, too.

[jcupitt]

Hi @manthey,

Thanks for reporting this. I think I thought those were pseudotags rather than real tags, so they were no longer necessary now we copy compressed tiles. You're right, they should go back.

I'll add a test for this case as well.

[jcupitt]

It seems TIFFTAG_PREDICTOR is the only non-pseudotag there, so we can just add a CopyField() for it. The other tags (webp, zstd) are pseudotags and cannot be copied (and don't need to be).

This will be in 8.15.2. I credited you in the changelog, I hope that's OK.

[jcupitt]

(and thanks again for reporting this dumb thing!)

[manthey]

Thank you.

[dgutman]

John thanks for responding / fixing this so quickly.. and your definition of dumb is not exactly the universe's definition of dumb.. :-)

[jcupitt]

You're right, the JCS_RGB flag probably shouldn't be set for non-RGB images. Nice!