Add more meaningful error message for 12-bit JPG files

Some Blackmagic CinemaDNG files (Pocket Cinema Camera 4K files (3)/(4), Micro Cinema Camera (1)) use SOF1 (Extended Sequential DCT) at 12-bit precision but label tiles as TIFF compression=7 (lossless JPEG). The lossless JPEG decoder hit a DQT marker and threw "Not a valid RAW file."

On libjpeg-turbo 2.1.5, the error is now "Unsupported JPEG data precision 12" (much clearer).

Future work: on systems with libjpeg-turbo 3.0+, we can enable full 12-bit lossy JPEG decoding.

Author: da-phil

src/librawspeed/decompressors/AbstractDngDecompressor.cpp

@@ -51,6 +51,64 @@
 
 namespace rawspeed {
 
+namespace {
+
+// Some DNG files (e.g. Blackmagic CinemaDNG) use TIFF compression=7
+// (lossless JPEG) but the actual tile data contains lossy DCT JPEG
+// (SOF0/SOF1/SOF2 with DQT). Detect this by scanning the first few
+// JPEG markers in the tile stream.
+[[nodiscard]] bool tileContainsLossyJpeg(const ByteStream& bs) {
+  const auto remaining = bs.getRemainSize();
+  if (remaining < 4)
+    return false;
+
+  // Must start with JPEG SOI marker
+  if (bs.peekByte(0) != 0xFF || bs.peekByte(1) != 0xD8)
+    return false;
+
+  // Scan markers after SOI. Stop after a reasonable number of bytes.
+  const auto limit =
+      std::min(remaining, static_cast<ByteStream::size_type>(1024));
+  ByteStream::size_type pos = 2;
+
+  while (pos + 3 < limit) {
+    if (bs.peekByte(pos) != 0xFF)
+      return false; // Invalid marker - stop scanning
+
+    const uint8_t marker = bs.peekByte(pos + 1);
+
+    // DQT (quantization table) is definitive proof of lossy JPEG
+    if (marker == 0xDB) // DQT
+      return true;
+
+    // SOF0/SOF1/SOF2 = lossy DCT-based JPEG
+    if (marker == 0xC0 || marker == 0xC1 || marker == 0xC2)
+      return true;
+
+    // SOF3 = lossless - this is what compression=7 should be
+    if (marker == 0xC3)
+      return false;
+
+    // SOS = start of scan data - stop scanning
+    if (marker == 0xDA)
+      return false;
+
+    // Skip this marker segment
+    if (pos + 4 > remaining)
+      return false;
+    const auto segLen = static_cast<uint16_t>(
+        (static_cast<uint16_t>(bs.peekByte(pos + 2)) << 8) |
+        static_cast<uint16_t>(bs.peekByte(pos + 3)));
+    if (segLen < 2)
+      return false;
+    pos += 2 + segLen;
+  }
+
+  return false;
+}
+
+} // namespace
+
 template <> void AbstractDngDecompressor::decompressThread<1>() const noexcept {
 #ifdef HAVE_OPENMP
 #pragma omp for schedule(static)
@@ -116,6 +174,15 @@ template <> void AbstractDngDecompressor::decompressThread<7>() const noexcept {
   for (const auto& e :
        Array1DRef(slices.data(), implicit_cast<int>(slices.size()))) {
     try {
+#ifdef HAVE_JPEG
+      // Some cameras (e.g. Blackmagic CinemaDNG) mislabel lossy DCT JPEG
+      // tiles as compression=7 (lossless JPEG). Detect and redirect.
+      if (tileContainsLossyJpeg(e.bs)) {
+        JpegDecompressor j(e.bs.peekBuffer(e.bs.getRemainSize()), mRaw);
+        j.decode(e.offX, e.offY);
+        continue;
+      }
+#endif
       LJpegDecoder d(e.bs, mRaw);
       d.decode(e.offX, e.offY, e.width, e.height,
                iPoint2D(e.dsc.tileW, e.dsc.tileH), mFixLjpeg);

src/librawspeed/decompressors/JpegDecompressor.cpp

@@ -139,6 +139,10 @@ void JpegDecompressor::decode(uint32_t offX,
   if (JPEG_HEADER_OK != jpeg_read_header(&dinfo, static_cast<boolean>(true)))
     ThrowRDE("Unable to read JPEG header");
 
+  if (dinfo.data_precision != 8)
+    ThrowRDE("Lossy JPEG tiles with %d-bit precision are not yet supported.",
+             dinfo.data_precision);
+
   jpeg_start_decompress(&dinfo);
   if (dinfo.output_components != static_cast<int>(mRaw->getCpp()))
     ThrowRDE("Component count doesn't match");