Allow disabling default emission of JPEG APP0 and APP14 segments

When embedding JPEGs into a container file format, it may be desirable
to minimize JPEG metadata, since the container will include the pertinent
details.  By default, libjpeg emits a JFIF APP0 segment for JFIF-
compatible colorspaces (grayscale or YCbCr) and Adobe APP14 otherwise.
Add a no_default_app_segments option to disable these.

660894cd36 added code to force emission of the JFIF segment if the DPI is
specified, even for JFIF-incompatible colorspaces.  This seems
inconsistent with the JFIF spec, but apparently other software does it
too.  no_default_app_segments does not disable this behavior, since it
only happens when the application explicitly specifies the DPI.

Tests/test_file_jpeg.py

@@ -88,6 +88,31 @@ class TestFileJpeg:
             assert im.info["comment"] == b"File written by Adobe Photoshop\xa8 4.0\x00"
             assert im.app["COM"] == im.info["comment"]
 
+    @pytest.mark.parametrize(
+        "keep_rgb, no_default_app_segments, expect_app0, expect_app14",
+        (
+            (False, False, True, False),
+            (True, False, False, True),
+            (False, True, False, False),
+            (True, True, False, False),
+        ),
+    )
+    def test_default_app_write(
+        self,
+        keep_rgb,
+        no_default_app_segments,
+        expect_app0,
+        expect_app14,
+    ):
+        im = self.roundtrip(
+            hopper(),
+            keep_rgb=keep_rgb,
+            no_default_app_segments=no_default_app_segments,
+        )
+        markers = {m[0] for m in im.applist}
+        assert ("APP0" in markers) == expect_app0
+        assert ("APP14" in markers) == expect_app14
+
     def test_comment_write(self):
         with Image.open(TEST_FILE) as im:
             assert im.info["comment"] == b"File written by Adobe Photoshop\xa8 4.0\x00"

docs/handbook/image-file-formats.rst

@@ -487,6 +487,13 @@ The :py:meth:`~PIL.Image.Image.save` method supports the following options:
 **exif**
     If present, the image will be stored with the provided raw EXIF data.
 
+**no_default_app_segments**
+    If present and true, the image is stored without default JFIF and Adobe
+    application segments.  The JFIF segment will still be stored if **dpi**
+    is also specified.
+
+    .. versionadded:: 10.3.0
+
 **keep_rgb**
     By default, libjpeg converts images with an RGB color space to YCbCr.
     If this option is present and true, those images will be stored as RGB

docs/releasenotes/10.3.0.rst

@@ -26,10 +26,12 @@ TODO
 API Additions
 =============
 
-TODO
-^^^^
+JPEG app segments
+^^^^^^^^^^^^^^^^^
 
-TODO
+When saving JPEG files, ``no_default_app_segments`` can now be set to ``True`` to store
+the image without default JFIF and Adobe application segments.  The JFIF segment will
+still be stored if ``dpi`` is also specified.
 
 Security
 ========

src/PIL/JpegImagePlugin.py

@@ -786,6 +786,7 @@ def _save(im, fp, filename):
         info.get("smooth", 0),
         optimize,
         info.get("keep_rgb", False),
+        info.get("no_default_app_segments", False),
         info.get("streamtype", 0),
         dpi[0],
         dpi[1],

src/encode.c

@@ -1043,6 +1043,7 @@ PyImaging_JpegEncoderNew(PyObject *self, PyObject *args) {
     Py_ssize_t smooth = 0;
     Py_ssize_t optimize = 0;
     int keep_rgb = 0;
+    int no_default_app_segments = 0;
     Py_ssize_t streamtype = 0; /* 0=interchange, 1=tables only, 2=image only */
     Py_ssize_t xdpi = 0, ydpi = 0;
     Py_ssize_t subsampling = -1; /* -1=default, 0=none, 1=medium, 2=high */
@@ -1060,7 +1061,7 @@ PyImaging_JpegEncoderNew(PyObject *self, PyObject *args) {
 
     if (!PyArg_ParseTuple(
             args,
-            "ss|nnnnpnnnnnnOz#y#y#",
+            "ss|nnnnppnnnnnnOz#y#y#",
             &mode,
             &rawmode,
             &quality,
@@ -1068,6 +1069,7 @@ PyImaging_JpegEncoderNew(PyObject *self, PyObject *args) {
             &smooth,
             &optimize,
             &keep_rgb,
+            &no_default_app_segments,
             &streamtype,
             &xdpi,
             &ydpi,
@@ -1153,6 +1155,7 @@ PyImaging_JpegEncoderNew(PyObject *self, PyObject *args) {
     strncpy(((JPEGENCODERSTATE *)encoder->state.context)->rawmode, rawmode, 8);
 
     ((JPEGENCODERSTATE *)encoder->state.context)->keep_rgb = keep_rgb;
+    ((JPEGENCODERSTATE *)encoder->state.context)->no_default_app_segments = no_default_app_segments;
     ((JPEGENCODERSTATE *)encoder->state.context)->quality = quality;
     ((JPEGENCODERSTATE *)encoder->state.context)->qtables = qarrays;
     ((JPEGENCODERSTATE *)encoder->state.context)->qtablesLen = qtablesLen;

src/libImaging/Jpeg.h

@@ -77,6 +77,9 @@ typedef struct {
     /* Disable automatic conversion of RGB images to YCbCr if non-zero */
     int keep_rgb;
 
+    /* Disable default application segments if non-zero */
+    int no_default_app_segments;
+
     /* Stream type (0=full, 1=tables only, 2=image only) */
     int streamtype;
 

src/libImaging/JpegEncode.c

@@ -161,6 +161,13 @@ ImagingJpegEncode(Imaging im, ImagingCodecState state, UINT8 *buf, int bytes) {
                 }
             }
 
+            /* Disable app markers if the colorspace enabled them.
+               xdpi/ydpi will still override this. */
+            if (context->no_default_app_segments) {
+                context->cinfo.write_JFIF_header = FALSE;
+                context->cinfo.write_Adobe_marker = FALSE;
+            }
+
             /* Use custom quantization tables */
             if (context->qtables) {
                 int i;