feat: Add metadata removal functionality

Dotnet/ScreenshotMetadata/PNGChunk.cs

@@ -1,6 +1,7 @@
 using System;
 using System.Buffers.Binary;
 using System.Collections.Generic;
+using System.IO;
 using System.Linq;
 using System.Text;
 
@@ -117,6 +118,35 @@ public struct PNGChunk
         return new Tuple<int, int>(width, height);
     }
 
+    /// <summary>
+    /// Validates this chunk against the chunk at the same index in a given file stream, checking the chunk length and CRC.
+    /// </summary>
+    /// <param name="fileStream">The file stream from which the chunk data is read for validation.</param>
+    /// <returns>True if chunk exists at index and is valid, false otherwise.</returns>
+    public bool ExistsInFile(FileStream fileStream)
+    {
+        fileStream.Seek(Index, SeekOrigin.Begin);
+        byte[] buffer = new byte[Length];
+        fileStream.ReadExactly(buffer, 0, Length);
+        
+        if (BitConverter.IsLittleEndian)
+            Array.Reverse(buffer, 0, 4);
+        
+        int chunkLength = BitConverter.ToInt32(buffer, 0);
+        if (chunkLength != Length)
+            return false;
+
+        fileStream.Seek(4 + chunkLength, SeekOrigin.Current);
+        fileStream.ReadExactly(buffer, 0, Length);
+        
+        if (BitConverter.IsLittleEndian)
+            Array.Reverse(buffer, 0, 4);
+        
+        uint crc = BitConverter.ToUInt32(buffer, 0);
+        
+        return crc == CalculateCRC();
+    }
+
     /// <summary>
     /// Constructs and returns a byte array representation of the PNG chunk. Generates a CRC.
     /// This data can be added to a PNG file as-is.
@@ -146,7 +176,7 @@ public struct PNGChunk
         Buffer.BlockCopy(Data, 0, result, 8, Data.Length);
     
         // Calculate and copy CRC
-        uint crc = Crc32(Data, 0, Data.Length, Crc32(chunkTypeBytes, 0, chunkTypeBytes.Length, 0));
+        uint crc = CalculateCRC();
         uint reversedCrc = BinaryPrimitives.ReverseEndianness(crc);
         
         Buffer.BlockCopy(BitConverter.GetBytes(reversedCrc), 0, result, totalLength - 4, 4);
@@ -154,6 +184,12 @@ public struct PNGChunk
         return result;
     }
 
+    public uint CalculateCRC()
+    {
+        var chunkTypeBytes = Encoding.ASCII.GetBytes(ChunkType);
+        return Crc32(Data, 0, Data.Length, Crc32(chunkTypeBytes, 0, chunkTypeBytes.Length, 0));
+    }
+
     // Crc32 implementation from
     // https://web.archive.org/web/20150825201508/http://upokecenter.dreamhosters.com/articles/png-image-encoder-in-c/
     private static uint Crc32(byte[] stream, int offset, int length, uint crc)

Dotnet/ScreenshotMetadata/PNGFile.cs

@@ -28,6 +28,11 @@ public class PNGFile : IDisposable
         fileStream = new FileStream(filePath, FileMode.Open, FileAccess.ReadWrite, FileShare.ReadWrite, 4096);
     }
     
+    public PNGFile(string filePath, int bufferSize)
+    {
+        fileStream = new FileStream(filePath, FileMode.Open, FileAccess.ReadWrite, FileShare.ReadWrite, bufferSize);
+    }
+
     /// <summary>
     /// Retrieves the first PNG chunk of the specified type from the file, or null if none were found.
     /// </summary>
@@ -107,6 +112,56 @@ public class PNGFile : IDisposable
         return true;
     }
 
+    /// <summary>
+    /// Deletes a PNG chunk from the file.
+    /// </summary>
+    /// <param name="chunk">The PNG chunk to delete. Needs a valid index set.</param>
+    /// <returns>True if the chunk was successfully deleted, otherwise false.</returns>
+    public bool DeleteChunk(PNGChunk chunk)
+    {
+        if (!chunk.ExistsInFile(fileStream))
+            return false;
+        
+        int bufferSize = 128 * 1024;
+        int deleteStart = chunk.Index;
+        int deleteLength = chunk.Length + CHUNK_NONDATA_SIZE;
+
+        long sourcePos = deleteStart + deleteLength;
+        long destPos = deleteStart;
+        byte[] buffer = new byte[bufferSize];
+
+        // Copy everything after the deleted section forward
+        while (sourcePos < fileStream.Length)
+        {
+            fileStream.Seek(sourcePos, SeekOrigin.Begin);
+            int bytesRead = fileStream.Read(buffer, 0, Math.Min(buffer.Length, (int)(fileStream.Length - sourcePos)));
+
+            if (bytesRead == 0)
+                break;
+
+            fileStream.Seek(destPos, SeekOrigin.Begin);
+            fileStream.Write(buffer, 0, bytesRead);
+
+            sourcePos += bytesRead;
+            destPos += bytesRead;
+        }
+
+        fileStream.SetLength(fileStream.Length - deleteLength);
+        
+        metadataChunkCache.Remove(chunk);
+        
+        // update the index of all cached chunks
+        for (int i = 0; i < metadataChunkCache.Count; i++)
+        {
+            var cachedChunk = metadataChunkCache[i];
+            if (cachedChunk.Index > deleteStart)
+                cachedChunk.Index -= deleteLength;
+        }
+
+        return true;
+    }
+    
+
     /// <summary>
     /// Retrieves all PNG metadata chunks of a specified type
     /// </summary>

Dotnet/ScreenshotMetadata/PNGHelper.cs

@@ -66,6 +66,22 @@ namespace VRCX
             return null;
         }
 
+        public static bool DeleteTextChunk(string keyword, PNGFile pngFile)
+        {
+            var iTXtChunk = pngFile.GetChunksOfType(PNGChunkTypeFilter.iTXt);
+            if (iTXtChunk.Count == 0)
+                return false;
+
+            for (int i = 0; i < iTXtChunk.Count; i++)
+            {
+                var data = iTXtChunk[i].ReadITXtChunk();
+                if (data.Item1 == keyword)
+                    return pngFile.DeleteChunk(iTXtChunk[i]);
+            }
+
+            return false;
+        }
+
         /// <summary>
         /// Generates a PNG text chunk ready for writing.
         /// </summary>

Dotnet/ScreenshotMetadata/ScreenshotHelper.cs

@@ -220,6 +220,15 @@ namespace VRCX
             return result;
         }
 
+        public static void DeleteTextMetadata(string path, bool deleteVRChatMetadata = false)
+        {
+            using var pngFile = new PNGFile(path, 128 * 1024);
+            if (deleteVRChatMetadata)
+                PNGHelper.DeleteTextChunk("XML:com.adobe.xmp", pngFile);
+            
+            PNGHelper.DeleteTextChunk("Description", pngFile);
+        }
+        
         public static bool WriteVRCXMetadata(string text, string path)
         {
             using var pngFile = new PNGFile(path);