Merge pull request #286 from drewnoakes/png-chunk-efficiency

Reduce allocation when reading PNG chunks
drewnoakes · May 7, 2021 · cdf9427 · cdf9427
2 parents 7f52477 + a131ad5
commit cdf9427
Showing 1 changed file with 19 additions and 2 deletions.
diff --git a/MetadataExtractor/Formats/Png/PngChunkReader.cs b/MetadataExtractor/Formats/Png/PngChunkReader.cs
@@ -48,6 +48,12 @@ public IEnumerable<PngChunk> Extract(SequentialReader reader, ICollection<PngChu
             //     Miscellaneous information: bKGD, hIST, pHYs, sPLT
             //     Time information:          tIME
             //
+            // CHUNK READING
+            //
+            //   Only chunk data for types specified in desiredChunkTypes is extracted.
+            //   For empty chunk type list NO data is copied from source stream.
+            //   For null chunk type list ALL data is copied from source stream.
+            //
 
             // network byte order
             reader = reader.WithByteOrder(isMotorolaByteOrder: true);
@@ -68,7 +74,17 @@ public IEnumerable<PngChunk> Extract(SequentialReader reader, ICollection<PngChu
                     throw new PngProcessingException("PNG chunk length exceeds maximum");
                 var chunkType = new PngChunkType(reader.GetBytes(4));
                 var willStoreChunk = desiredChunkTypes == null || desiredChunkTypes.Contains(chunkType);
-                var chunkData = reader.GetBytes(chunkDataLength);
+
+                byte[]? chunkData;
+                if (willStoreChunk)
+                {
+                    chunkData = reader.GetBytes(chunkDataLength);
+                }
+                else
+                {
+                    chunkData = null;
+                    reader.Skip(chunkDataLength);
+                }
 
                 // Skip the CRC bytes at the end of the chunk
                 // TODO consider verifying the CRC value to determine if we're processing bad data
@@ -85,7 +101,8 @@ public IEnumerable<PngChunk> Extract(SequentialReader reader, ICollection<PngChu
                 if (chunkType.Equals(PngChunkType.IEND))
                     seenImageTrailer = true;
 
-                if (willStoreChunk)
+                // chunkData will be null if we aren't interested in this chunk
+                if (chunkData is not null)
                     chunks.Add(new PngChunk(chunkType, chunkData));
 
                 seenChunkTypes.Add(chunkType);