Support writing QOI images (#9007)

Co-authored-by: Andrew Murray <radarhere@users.noreply.github.com>

Tests/test_file_qoi.py

@@ -1,10 +1,12 @@
 from __future__ import annotations
 
+from pathlib import Path
+
 import pytest
 
 from PIL import Image, QoiImagePlugin
 
-from .helper import assert_image_equal_tofile
+from .helper import assert_image_equal_tofile, hopper
 
 
 def test_sanity() -> None:
@@ -34,3 +36,22 @@ def test_op_index() -> None:
     # QOI_OP_INDEX as the first chunk
     with Image.open("Tests/images/op_index.qoi") as im:
         assert im.getpixel((0, 0)) == (0, 0, 0, 0)
+
+
+def test_save(tmp_path: Path) -> None:
+    f = tmp_path / "temp.qoi"
+
+    im = hopper()
+    im.save(f, colorspace="sRGB")
+
+    assert_image_equal_tofile(im, f)
+
+    for path in ("Tests/images/default_font.png", "Tests/images/pil123rgba.png"):
+        with Image.open(path) as im:
+            im.save(f)
+
+            assert_image_equal_tofile(im, f)
+
+    im = hopper("P")
+    with pytest.raises(ValueError, match="Unsupported QOI image mode"):
+        im.save(f)

docs/handbook/image-file-formats.rst

@@ -1082,6 +1082,26 @@ Pillow reads and writes PBM, PGM, PPM and PNM files containing ``1``, ``L``, ``I
 
 Since Pillow 9.2.0, "plain" (P1 to P3) formats can be read as well.
 
+QOI
+^^^
+
+.. versionadded:: 9.5.0
+
+Pillow reads and writes images in Quite OK Image format using a Python codec. If you
+wish to write code specifically for this format, :pypi:`qoi` is an alternative library
+that uses C to decode the image and interfaces with NumPy.
+
+.. _qoi-saving:
+
+Saving
+~~~~~~
+
+The :py:meth:`~PIL.Image.Image.save` method can take the following keyword arguments:
+
+**colorspace**
+    If set to "sRGB", the colorspace will be written as sRGB with linear alpha, instead
+    of all channels being linear.
+
 SGI
 ^^^
 
@@ -1578,15 +1598,6 @@ PSD
 
 Pillow identifies and reads PSD files written by Adobe Photoshop 2.5 and 3.0.
 
-QOI
-^^^
-
-.. versionadded:: 9.5.0
-
-Pillow reads images in Quite OK Image format using a Python decoder. If you wish to
-write code specifically for this format, :pypi:`qoi` is an alternative library that
-uses C to decode the image and interfaces with NumPy.
-
 SUN
 ^^^
 

docs/releasenotes/11.3.0.rst

@@ -47,6 +47,13 @@ TODO
 Other changes
 =============
 
+Added QOI saving
+^^^^^^^^^^^^^^^^
+
+Support has been added for saving QOI images. ``colorspace`` can be used to specify the
+colorspace as sRGB with linear alpha, e.g. ``im.save("out.qoi", colorspace="sRGB")``.
+By default, all channels will be linear.
+
 Support using more screenshot utilities with ImageGrab on Linux
 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
 

src/PIL/QoiImagePlugin.py

@@ -8,9 +8,12 @@
 from __future__ import annotations
 
 import os
+from typing import IO
 
 from . import Image, ImageFile
 from ._binary import i32be as i32
+from ._binary import o8
+from ._binary import o32be as o32
 
 
 def _accept(prefix: bytes) -> bool:
@@ -110,6 +113,122 @@ class QoiDecoder(ImageFile.PyDecoder):
         return -1, 0
 
 
+def _save(im: Image.Image, fp: IO[bytes], filename: str | bytes) -> None:
+    if im.mode == "RGB":
+        channels = 3
+    elif im.mode == "RGBA":
+        channels = 4
+    else:
+        msg = "Unsupported QOI image mode"
+        raise ValueError(msg)
+
+    colorspace = 0 if im.encoderinfo.get("colorspace") == "sRGB" else 1
+
+    fp.write(b"qoif")
+    fp.write(o32(im.size[0]))
+    fp.write(o32(im.size[1]))
+    fp.write(o8(channels))
+    fp.write(o8(colorspace))
+
+    ImageFile._save(im, fp, [ImageFile._Tile("qoi", (0, 0) + im.size)])
+
+
+class QoiEncoder(ImageFile.PyEncoder):
+    _pushes_fd = True
+    _previous_pixel: tuple[int, int, int, int] | None = None
+    _previously_seen_pixels: dict[int, tuple[int, int, int, int]] = {}
+    _run = 0
+
+    def _write_run(self) -> bytes:
+        data = o8(0b11000000 | (self._run - 1))  # QOI_OP_RUN
+        self._run = 0
+        return data
+
+    def _delta(self, left: int, right: int) -> int:
+        result = (left - right) & 255
+        if result >= 128:
+            result -= 256
+        return result
+
+    def encode(self, bufsize: int) -> tuple[int, int, bytes]:
+        assert self.im is not None
+
+        self._previously_seen_pixels = {0: (0, 0, 0, 0)}
+        self._previous_pixel = (0, 0, 0, 255)
+
+        data = bytearray()
+        w, h = self.im.size
+        bands = Image.getmodebands(self.mode)
+
+        for y in range(h):
+            for x in range(w):
+                pixel = self.im.getpixel((x, y))
+                if bands == 3:
+                    pixel = (*pixel, 255)
+
+                if pixel == self._previous_pixel:
+                    self._run += 1
+                    if self._run == 62:
+                        data += self._write_run()
+                else:
+                    if self._run:
+                        data += self._write_run()
+
+                    r, g, b, a = pixel
+                    hash_value = (r * 3 + g * 5 + b * 7 + a * 11) % 64
+                    if self._previously_seen_pixels.get(hash_value) == pixel:
+                        data += o8(hash_value)  # QOI_OP_INDEX
+                    elif self._previous_pixel:
+                        self._previously_seen_pixels[hash_value] = pixel
+
+                        prev_r, prev_g, prev_b, prev_a = self._previous_pixel
+                        if prev_a == a:
+                            delta_r = self._delta(r, prev_r)
+                            delta_g = self._delta(g, prev_g)
+                            delta_b = self._delta(b, prev_b)
+
+                            if (
+                                -2 <= delta_r < 2
+                                and -2 <= delta_g < 2
+                                and -2 <= delta_b < 2
+                            ):
+                                data += o8(
+                                    0b01000000
+                                    | (delta_r + 2) << 4
+                                    | (delta_g + 2) << 2
+                                    | (delta_b + 2)
+                                )  # QOI_OP_DIFF
+                            else:
+                                delta_gr = self._delta(delta_r, delta_g)
+                                delta_gb = self._delta(delta_b, delta_g)
+                                if (
+                                    -8 <= delta_gr < 8
+                                    and -32 <= delta_g < 32
+                                    and -8 <= delta_gb < 8
+                                ):
+                                    data += o8(
+                                        0b10000000 | (delta_g + 32)
+                                    )  # QOI_OP_LUMA
+                                    data += o8((delta_gr + 8) << 4 | (delta_gb + 8))
+                                else:
+                                    data += o8(0b11111110)  # QOI_OP_RGB
+                                    data += bytes(pixel[:3])
+                        else:
+                            data += o8(0b11111111)  # QOI_OP_RGBA
+                            data += bytes(pixel)
+
+                self._previous_pixel = pixel
+
+        if self._run:
+            data += self._write_run()
+        data += bytes((0, 0, 0, 0, 0, 0, 0, 1))  # padding
+
+        return len(data), 0, data
+
+
 Image.register_open(QoiImageFile.format, QoiImageFile, _accept)
 Image.register_decoder("qoi", QoiDecoder)
 Image.register_extension(QoiImageFile.format, ".qoi")
+
+Image.register_save(QoiImageFile.format, _save)
+Image.register_encoder("qoi", QoiEncoder)