1 files changed, 61 insertions, 0 deletions
diff --git a/lib/extras/common.cc b/lib/extras/common.cc
new file mode 100644
index 0000000..e85b43a
--- /dev/null
+++ b/lib/extras/common.cc
@@ -0,0 +1,61 @@
+// Copyright (c) the JPEG XL Project Authors. All rights reserved.
+//
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+#include "lib/extras/common.h"
+
+#include <jxl/codestream_header.h>
+#include <jxl/types.h>
+
+#include <cstddef>
+#include <vector>
+
+#include "lib/extras/packed_image.h"
+#include "lib/jxl/base/printf_macros.h"
+#include "lib/jxl/base/status.h"
+
+namespace jxl {
+namespace extras {
+
+Status SelectFormat(const std::vector<JxlPixelFormat>& accepted_formats,
+                    const JxlBasicInfo& basic_info, JxlPixelFormat* format) {
+  const size_t original_bit_depth = basic_info.bits_per_sample;
+  size_t current_bit_depth = 0;
+  size_t num_alpha_channels = (basic_info.alpha_bits != 0 ? 1 : 0);
+  size_t num_channels = basic_info.num_color_channels + num_alpha_channels;
+  for (;;) {
+    for (const JxlPixelFormat& candidate : accepted_formats) {
+      if (candidate.num_channels != num_channels) continue;
+      const size_t candidate_bit_depth =
+          PackedImage::BitsPerChannel(candidate.data_type);
+      if (
+          // Candidate bit depth is less than what we have and still enough
+          (original_bit_depth <= candidate_bit_depth &&
+           candidate_bit_depth < current_bit_depth) ||
+          // Or larger than the too-small bit depth we currently have
+          (current_bit_depth < candidate_bit_depth &&
+           current_bit_depth < original_bit_depth)) {
+        *format = candidate;
+        current_bit_depth = candidate_bit_depth;
+      }
+    }
+    if (current_bit_depth == 0) {
+      if (num_channels > basic_info.num_color_channels) {
+        // Try dropping the alpha channel.
+        --num_channels;
+        continue;
+      }
+      return JXL_FAILURE("no appropriate format found");
+    }
+    break;
+  }
+  if (current_bit_depth < original_bit_depth) {
+    JXL_WARNING("encoding %" PRIuS "-bit original to %" PRIuS " bits",
+                original_bit_depth, current_bit_depth);
+  }
+  return true;
+}
+
+}  // namespace extras
+}  // namespace jxl