tiff: add support for JPEG quality

Uses JPEGQUALITY pseudo-tag from libtiff.

Also changes the way tags are passed to PyImaging_LibTiffEncoderNew from
dict to list to ensure that COMPRESSION tag is added before JPEGQUALITY.
This is required as the COMPRESSION tag registers the JPEGQUALITY
pseudo-tag.

Tests/test_file_libtiff.py

@@ -435,18 +435,44 @@ class TestFileLibTiff(LibTiffTestCase):
         self.assert_image_equal(im, im2)
 
     def test_compressions(self):
+        # Test various tiff compressions and assert similar image content but reduced
+        # file sizes.
         im = hopper("RGB")
         out = self.tempfile("temp.tif")
+        im.save(out)
+        size_raw = os.path.getsize(out)
 
         for compression in ("packbits", "tiff_lzw"):
             im.save(out, compression=compression)
+            size_compressed = os.path.getsize(out)
             im2 = Image.open(out)
             self.assert_image_equal(im, im2)
 
         im.save(out, compression="jpeg")
+        size_jpeg = os.path.getsize(out)
         im2 = Image.open(out)
         self.assert_image_similar(im, im2, 30)
 
+        im.save(out, compression="jpeg", quality=30)
+        size_jpeg_30 = os.path.getsize(out)
+        im3 = Image.open(out)
+        self.assert_image_similar(im2, im3, 30)
+
+        assert size_raw > size_compressed
+        assert size_compressed > size_jpeg
+        assert size_jpeg > size_jpeg_30
+
+    def test_quality(self):
+        im = hopper("RGB")
+        out = self.tempfile("temp.tif")
+
+        self.assertRaises(ValueError, im.save, out, compression="tiff_lzw", quality=50)
+        self.assertRaises(ValueError, im.save, out, compression="jpeg", quality=-1)
+        self.assertRaises(ValueError, im.save, out, compression="jpeg", quality=101)
+        self.assertRaises(ValueError, im.save, out, compression="jpeg", quality="good")
+        im.save(out, compression="jpeg", quality=0)
+        im.save(out, compression="jpeg", quality=100)
+
     def test_cmyk_save(self):
         im = hopper("CMYK")
         out = self.tempfile("temp.tif")

docs/handbook/image-file-formats.rst

@@ -738,6 +738,12 @@ The :py:meth:`~PIL.Image.Image.save` method can take the following keyword argum
     ``"tiff_thunderscan"``, ``"tiff_deflate"``, ``"tiff_sgilog"``,
     ``"tiff_sgilog24"``, ``"tiff_raw_16"``
 
+**quality**
+    The image quality for JPEG compression, on a scale from 0 (worst) to 100
+    (best). The default is 75.
+
+    .. versionadded:: 6.1.0
+
 These arguments to set the tiff header fields are an alternative to
 using the general tags available through tiffinfo.
 

src/PIL/TiffImagePlugin.py

@@ -116,6 +116,7 @@ PHOTOSHOP_CHUNK = 34377  # photoshop properties
 ICCPROFILE = 34675
 EXIFIFD = 34665
 XMP = 700
+JPEGQUALITY = 65537  # pseudo-tag by libtiff
 
 # https://github.com/imagej/ImageJA/blob/master/src/main/java/ij/io/TiffDecoder.java
 IMAGEJ_META_DATA_BYTE_COUNTS = 50838
@@ -1529,6 +1530,16 @@ def _save(im, fp, filename):
     ifd[COMPRESSION] = COMPRESSION_INFO_REV.get(compression, 1)
 
     if libtiff:
+        if "quality" in im.encoderinfo:
+            quality = im.encoderinfo["quality"]
+            if not isinstance(quality, int) or quality < 0 or quality > 100:
+                raise ValueError("Invalid quality setting")
+            if compression != "jpeg":
+                raise ValueError(
+                    "quality setting only supported for 'jpeg' compression"
+                )
+            ifd[JPEGQUALITY] = quality
+
         if DEBUG:
             print("Saving using libtiff encoder")
             print("Items: %s" % sorted(ifd.items()))
@@ -1604,7 +1615,12 @@ def _save(im, fp, filename):
         if im.mode in ("I;16B", "I;16"):
             rawmode = "I;16N"
 
-        a = (rawmode, compression, _fp, filename, atts, types)
+        # Pass tags as sorted list so that the tags are set in a fixed order.
+        # This is required by libtiff for some tags. For example, the JPEGQUALITY
+        # pseudo tag requires that the COMPRESS tag was already set.
+        tags = list(atts.items())
+        tags.sort()
+        a = (rawmode, compression, _fp, filename, tags, types)
         e = Image._getencoder(im.mode, "libtiff", a, im.encoderconfig)
         e.setimage(im.im, (0, 0) + im.size)
         while True:

src/PIL/TiffTags.py

@@ -432,6 +432,9 @@ TYPES = {}
 # 389: case TIFFTAG_REFERENCEBLACKWHITE:
 # 393: case TIFFTAG_INKNAMES:
 
+# Following pseudo-tags are also handled by default in libtiff:
+# TIFFTAG_JPEGQUALITY 65537
+
 # some of these are not in our TAGS_V2 dict and were included from tiff.h
 
 # This list also exists in encode.c
@@ -476,6 +479,7 @@ LIBTIFF_CORE = {
     333,
     # as above
     269,  # this has been in our tests forever, and works
+    65537,
 }
 
 LIBTIFF_CORE.remove(320)  # Array of short, crashes

src/encode.c

@@ -643,27 +643,29 @@ PyImaging_LibTiffEncoderNew(PyObject* self, PyObject* args)
     const int core_tags[] = {
         256, 257, 258, 259, 262, 263, 266, 269, 274, 277, 278, 280, 281, 340,
         341, 282, 283, 284, 286, 287, 296, 297, 321, 338, 32995, 32998, 32996,
-        339, 32997, 330, 531, 530
+        339, 32997, 330, 531, 530, 65537
     };
 
-    Py_ssize_t d_size;
-    PyObject *keys, *values;
+    Py_ssize_t tags_size;
+    PyObject *item;
 
 
     if (! PyArg_ParseTuple(args, "sssisOO", &mode, &rawmode, &compname, &fp, &filename, &tags, &types)) {
         return NULL;
     }
 
-    if (!PyDict_Check(tags)) {
-        PyErr_SetString(PyExc_ValueError, "Invalid tags dictionary");
+    if (!PyList_Check(tags)) {
+        PyErr_SetString(PyExc_ValueError, "Invalid tags list");
         return NULL;
     } else {
-        d_size = PyDict_Size(tags);
-        TRACE(("dict size: %d\n", (int)d_size));
-        keys = PyDict_Keys(tags);
-        values = PyDict_Values(tags);
-        for (pos=0;pos<d_size;pos++){
-            TRACE(("  key: %d\n", (int)PyInt_AsLong(PyList_GetItem(keys,pos))));
+        tags_size = PyList_Size(tags);
+        TRACE(("tags size: %d\n", (int)tags_size));
+        for (pos=0;pos<tags_size;pos++){
+            item = PyList_GetItem(tags, pos);
+            if (!PyTuple_Check(item) || PyTuple_Size(item) != 2) {
+               PyErr_SetString(PyExc_ValueError, "Invalid tags list");
+               return NULL;
+            }
         }
         pos = 0;
     }
@@ -688,10 +690,12 @@ PyImaging_LibTiffEncoderNew(PyObject* self, PyObject* args)
     }
 
     num_core_tags = sizeof(core_tags) / sizeof(int);
-    for (pos = 0; pos < d_size; pos++) {
-        key = PyList_GetItem(keys, pos);
+    for (pos = 0; pos < tags_size; pos++) {
+        item = PyList_GetItem(tags, pos);
+        // We already checked that tags is a 2-tuple list.
+        key = PyTuple_GetItem(item, 0);
         key_int = (int)PyInt_AsLong(key);
-        value = PyList_GetItem(values, pos);
+        value = PyTuple_GetItem(item, 1);
         status = 0;
         is_core_tag = 0;
         is_var_length = 0;