Allow configuring JPEG restart marker interval on save

Tests/test_file_jpeg.py

@@ -643,6 +643,23 @@ def test_save_low_quality_baseline_qtables(self):
             assert max(im2.quantization[0]) <= 255
             assert max(im2.quantization[1]) <= 255
 
+    @pytest.mark.parametrize(
+        "blocks, rows, markers",
+        ((0, 0, 0), (1, 0, 15), (3, 0, 5), (8, 0, 1), (0, 1, 3), (0, 2, 1)),
+    )
+    def test_restart_markers(self, blocks, rows, markers):
+        im = Image.new("RGB", (32, 32))  # 16 MCUs
+        out = BytesIO()
+        im.save(
+            out,
+            format="JPEG",
+            restart_marker_blocks=blocks,
+            restart_marker_rows=rows,
+            # force 8x8 pixel MCUs
+            subsampling=0,
+        )
+        assert len(re.findall(b"\xff[\xd0-\xd7]", out.getvalue())) == markers
+
     @pytest.mark.skipif(not djpeg_available(), reason="djpeg not available")
     def test_load_djpeg(self):
         with Image.open(TEST_FILE) as img:

docs/handbook/image-file-formats.rst

@@ -494,6 +494,18 @@ The :py:meth:`~PIL.Image.Image.save` method supports the following options:
 
     If absent, the setting will be determined by libjpeg or libjpeg-turbo.
 
+**restart_marker_blocks**
+    If present, emit a restart marker whenever the specified number of MCU
+    blocks has been produced.
+
+    .. versionadded:: 10.2.0
+
+**restart_marker_rows**
+    If present, emit a restart marker whenever the specified number of MCU
+    rows has been produced.
+
+    .. versionadded:: 10.2.0
+
 **qtables**
     If present, sets the qtables for the encoder. This is listed as an
     advanced option for wizards in the JPEG documentation. Use with

src/PIL/JpegImagePlugin.py

@@ -787,6 +787,8 @@ def validate_qtables(qtables):
         dpi[0],
         dpi[1],
         subsampling,
+        info.get("restart_marker_blocks", 0),
+        info.get("restart_marker_rows", 0),
         qtables,
         comment,
         extra,

src/encode.c

@@ -1045,6 +1045,8 @@ PyImaging_JpegEncoderNew(PyObject *self, PyObject *args) {
     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 */
+    Py_ssize_t restart_marker_blocks = 0;
+    Py_ssize_t restart_marker_rows = 0;
     PyObject *qtables = NULL;
     unsigned int *qarrays = NULL;
     int qtablesLen = 0;
@@ -1057,7 +1059,7 @@ PyImaging_JpegEncoderNew(PyObject *self, PyObject *args) {
 
     if (!PyArg_ParseTuple(
             args,
-            "ss|nnnnnnnnOz#y#y#",
+            "ss|nnnnnnnnnnOz#y#y#",
             &mode,
             &rawmode,
             &quality,
@@ -1068,6 +1070,8 @@ PyImaging_JpegEncoderNew(PyObject *self, PyObject *args) {
             &xdpi,
             &ydpi,
             &subsampling,
+            &restart_marker_blocks,
+            &restart_marker_rows,
             &qtables,
             &comment,
             &comment_size,
@@ -1156,6 +1160,8 @@ PyImaging_JpegEncoderNew(PyObject *self, PyObject *args) {
     ((JPEGENCODERSTATE *)encoder->state.context)->streamtype = streamtype;
     ((JPEGENCODERSTATE *)encoder->state.context)->xdpi = xdpi;
     ((JPEGENCODERSTATE *)encoder->state.context)->ydpi = ydpi;
+    ((JPEGENCODERSTATE *)encoder->state.context)->restart_marker_blocks = restart_marker_blocks;
+    ((JPEGENCODERSTATE *)encoder->state.context)->restart_marker_rows = restart_marker_rows;
     ((JPEGENCODERSTATE *)encoder->state.context)->comment = comment;
     ((JPEGENCODERSTATE *)encoder->state.context)->comment_size = comment_size;
     ((JPEGENCODERSTATE *)encoder->state.context)->extra = extra;

src/libImaging/Jpeg.h

@@ -83,6 +83,10 @@ typedef struct {
     /* Chroma Subsampling (-1=default, 0=none, 1=medium, 2=high) */
     int subsampling;
 
+    /* Restart marker interval, in MCU blocks or MCU rows, or 0 for none */
+    unsigned int restart_marker_blocks;
+    unsigned int restart_marker_rows;
+
     /* Converter input mode (input to the shuffler) */
     char rawmode[8 + 1];
 

src/libImaging/JpegEncode.c

@@ -210,6 +210,8 @@ ImagingJpegEncode(Imaging im, ImagingCodecState state, UINT8 *buf, int bytes) {
             }
             context->cinfo.smoothing_factor = context->smooth;
             context->cinfo.optimize_coding = (boolean)context->optimize;
+            context->cinfo.restart_interval = context->restart_marker_blocks;
+            context->cinfo.restart_in_rows = context->restart_marker_rows;
             if (context->xdpi > 0 && context->ydpi > 0) {
                 context->cinfo.write_JFIF_header = TRUE;
                 context->cinfo.density_unit = 1; /* dots per inch */