Added support for CMYK in decode_jpeg

test/test_image.py

@@ -83,12 +83,9 @@ def test_decode_jpeg(img_path, pil_mode, mode):
     with Image.open(img_path) as img:
         is_cmyk = img.mode == "CMYK"
         if pil_mode is not None:
-            if is_cmyk:
-                # libjpeg does not support the conversion
-                pytest.xfail("Decoding a CMYK jpeg isn't supported")
             img = img.convert(pil_mode)
         img_pil = torch.from_numpy(np.array(img))
-        if is_cmyk:
+        if is_cmyk and mode == ImageReadMode.UNCHANGED:
             # flip the colors to match libjpeg
             img_pil = 255 - img_pil
 

torchvision/csrc/io/image/cpu/decode_jpeg.cpp

@@ -67,6 +67,58 @@ static void torch_jpeg_set_source_mgr(
   src->pub.next_input_byte = src->data;
 }
 
+inline unsigned char clamped_cmyk_rgb_convert(
+    unsigned char k,
+    unsigned char cmy) {
+  // Inspired from Pillow:
+  // https://github.com/python-pillow/Pillow/blob/07623d1a7cc65206a5355fba2ae256550bfcaba6/src/libImaging/Convert.c#L568-L569
+  int v = k * cmy + 128;
+  v = ((v >> 8) + v) >> 8;
+  return std::clamp(k - v, 0, 255);
+}
+
+void convert_line_cmyk_to_rgb(
+    j_decompress_ptr cinfo,
+    const unsigned char* cmyk_line,
+    unsigned char* rgb_line) {
+  int width = cinfo->output_width;
+  for (int i = 0; i < width; ++i) {
+    int c = cmyk_line[i * 4 + 0];
+    int m = cmyk_line[i * 4 + 1];
+    int y = cmyk_line[i * 4 + 2];
+    int k = cmyk_line[i * 4 + 3];
+
+    rgb_line[i * 3 + 0] = clamped_cmyk_rgb_convert(k, 255 - c);
+    rgb_line[i * 3 + 1] = clamped_cmyk_rgb_convert(k, 255 - m);
+    rgb_line[i * 3 + 2] = clamped_cmyk_rgb_convert(k, 255 - y);
+  }
+}
+
+inline unsigned char rgb_to_gray(int r, int g, int b) {
+  // Inspired from Pillow:
+  // https://github.com/python-pillow/Pillow/blob/07623d1a7cc65206a5355fba2ae256550bfcaba6/src/libImaging/Convert.c#L226
+  return (r * 19595 + g * 38470 + b * 7471 + 0x8000) >> 16;
+}
+
+void convert_line_cmyk_to_gray(
+    j_decompress_ptr cinfo,
+    const unsigned char* cmyk_line,
+    unsigned char* gray_line) {
+  int width = cinfo->output_width;
+  for (int i = 0; i < width; ++i) {
+    int c = cmyk_line[i * 4 + 0];
+    int m = cmyk_line[i * 4 + 1];
+    int y = cmyk_line[i * 4 + 2];
+    int k = cmyk_line[i * 4 + 3];
+
+    int r = clamped_cmyk_rgb_convert(k, 255 - c);
+    int g = clamped_cmyk_rgb_convert(k, 255 - m);
+    int b = clamped_cmyk_rgb_convert(k, 255 - y);
+
+    gray_line[i] = rgb_to_gray(r, g, b);
+  }
+}
+
 } // namespace
 
 torch::Tensor decode_jpeg(const torch::Tensor& data, ImageReadMode mode) {
@@ -102,20 +154,29 @@ torch::Tensor decode_jpeg(const torch::Tensor& data, ImageReadMode mode) {
   jpeg_read_header(&cinfo, TRUE);
 
   int channels = cinfo.num_components;
+  bool cmyk_to_rgb_or_gray = false;
 
   if (mode != IMAGE_READ_MODE_UNCHANGED) {
     switch (mode) {
       case IMAGE_READ_MODE_GRAY:
-        if (cinfo.jpeg_color_space != JCS_GRAYSCALE) {
+        if (cinfo.jpeg_color_space == JCS_CMYK ||
+            cinfo.jpeg_color_space == JCS_YCCK) {
+          cinfo.out_color_space = JCS_CMYK;
+          cmyk_to_rgb_or_gray = true;
+        } else {
           cinfo.out_color_space = JCS_GRAYSCALE;
-          channels = 1;
         }
+        channels = 1;
         break;
       case IMAGE_READ_MODE_RGB:
-        if (cinfo.jpeg_color_space != JCS_RGB) {
+        if (cinfo.jpeg_color_space == JCS_CMYK ||
+            cinfo.jpeg_color_space == JCS_YCCK) {
+          cinfo.out_color_space = JCS_CMYK;
+          cmyk_to_rgb_or_gray = true;
+        } else {
           cinfo.out_color_space = JCS_RGB;
-          channels = 3;
         }
+        channels = 3;
         break;
       /*
        * Libjpeg does not support converting from CMYK to grayscale etc. There
@@ -139,12 +200,28 @@ torch::Tensor decode_jpeg(const torch::Tensor& data, ImageReadMode mode) {
   auto tensor =
       torch::empty({int64_t(height), int64_t(width), channels}, torch::kU8);
   auto ptr = tensor.data_ptr<uint8_t>();
+  torch::Tensor cmyk_line_tensor;
+  if (cmyk_to_rgb_or_gray) {
+    cmyk_line_tensor = torch::empty({int64_t(width), 4}, torch::kU8);
+  }
+
   while (cinfo.output_scanline < cinfo.output_height) {
     /* jpeg_read_scanlines expects an array of pointers to scanlines.
      * Here the array is only one element long, but you could ask for
      * more than one scanline at a time if that's more convenient.
      */
-    jpeg_read_scanlines(&cinfo, &ptr, 1);
+    if (cmyk_to_rgb_or_gray) {
+      auto cmyk_line_ptr = cmyk_line_tensor.data_ptr<uint8_t>();
+      jpeg_read_scanlines(&cinfo, &cmyk_line_ptr, 1);
+
+      if (channels == 3) {
+        convert_line_cmyk_to_rgb(&cinfo, cmyk_line_ptr, ptr);
+      } else if (channels == 1) {
+        convert_line_cmyk_to_gray(&cinfo, cmyk_line_ptr, ptr);
+      }
+    } else {
+      jpeg_read_scanlines(&cinfo, &ptr, 1);
+    }
     ptr += stride;
   }