Fix qtables and quality scaling (#8879)

Co-authored-by: Andrew Murray <[email protected]>

Author: Kyliroco

Tests/test_file_jpeg.py

@@ -611,6 +611,24 @@ def _n_qtables_helper(n: int, test_file: str) -> None:
                     None
                 )
             ]
+
+            for quality in range(101):
+                qtable_from_qtable_quality = self.roundtrip(
+                    im,
+                    qtables={0: standard_l_qtable, 1: standard_chrominance_qtable},
+                    quality=quality,
+                ).quantization
+
+                qtable_from_quality = self.roundtrip(im, quality=quality).quantization
+
+                if features.check_feature("libjpeg_turbo"):
+                    assert qtable_from_qtable_quality == qtable_from_quality
+                else:
+                    assert qtable_from_qtable_quality[0] == qtable_from_quality[0]
+                    assert (
+                        qtable_from_qtable_quality[1][1:] == qtable_from_quality[1][1:]
+                    )
+
             # list of qtable lists
             assert_image_similar(
                 im,

docs/handbook/image-file-formats.rst

@@ -557,6 +557,8 @@ The :py:meth:`~PIL.Image.Image.save` method supports the following options:
     hardly any gain in image quality. The value ``keep`` is only valid for JPEG
     files and will retain the original image quality level, subsampling, and
     qtables.
+    For more information on how qtables are modified based on the quality parameter,
+    see the qtables section.
 
 **optimize**
     If present and true, indicates that the encoder should make an extra pass
@@ -622,6 +624,11 @@ The :py:meth:`~PIL.Image.Image.save` method supports the following options:
        range(len(keys))) of lists of 64 integers. There must be
        between 2 and 4 tables.
 
+    If a quality parameter is provided, the qtables will be adjusted accordingly.
+    By default, the qtables are based on a standard JPEG table with a quality of 50.
+    The qtable values will be reduced if the quality is higher than 50 and increased
+    if the quality is lower than 50.
+
     .. versionadded:: 2.5.0
 
 **streamtype**

src/libImaging/JpegEncode.c

@@ -175,18 +175,21 @@ ImagingJpegEncode(Imaging im, ImagingCodecState state, UINT8 *buf, int bytes) {
             /* Use custom quantization tables */
             if (context->qtables) {
                 int i;
-                int quality = 100;
+                int quality = 50;
                 int last_q = 0;
+                boolean force_baseline = FALSE;
                 if (context->quality != -1) {
                     quality = context->quality;
+                    force_baseline = TRUE;
                 }
+                int scale_factor = jpeg_quality_scaling(quality);
                 for (i = 0; i < context->qtablesLen; i++) {
                     jpeg_add_quant_table(
                         &context->cinfo,
                         i,
                         &context->qtables[i * DCTSIZE2],
-                        quality,
-                        FALSE
+                        scale_factor,
+                        force_baseline
                     );
                     context->cinfo.comp_info[i].quant_tbl_no = i;
                     last_q = i;
@@ -195,7 +198,11 @@ ImagingJpegEncode(Imaging im, ImagingCodecState state, UINT8 *buf, int bytes) {
                     // jpeg_set_defaults created two qtables internally, but we only
                     // wanted one.
                     jpeg_add_quant_table(
-                        &context->cinfo, 1, &context->qtables[0], quality, FALSE
+                        &context->cinfo,
+                        1,
+                        &context->qtables[0],
+                        scale_factor,
+                        force_baseline
                     );
                 }
                 for (i = last_q; i < context->cinfo.num_components; i++) {