Fix encoding of 16 tga files

src/ImageSharp/Formats/Tga/TgaDecoderCore.cs

@@ -141,7 +141,6 @@ namespace SixLabors.ImageSharp.Formats.Tga
                     case 16:
                         if (this.fileHeader.ImageType.IsRunLengthEncoded())
                         {
-                            long currentPosition = this.currentStream.Position;
                             this.ReadRle(this.fileHeader.Width, this.fileHeader.Height, pixels, 2, inverted);
                         }
                         else
@@ -218,7 +217,7 @@ namespace SixLabors.ImageSharp.Formats.Tga
                             {
                                 int colorIndex = rowSpan[x];
 
-                                // Set bit 16 to 1, to treat it as opaque for Bgra5551.
+                                // Set alpha value to 1, to treat it as opaque for Bgra5551.
                                 Bgra5551 bgra = Unsafe.As<byte, Bgra5551>(ref palette[colorIndex * colorMapPixelSizeInBytes]);
                                 bgra.PackedValue = (ushort)(bgra.PackedValue | 0x8000);
                                 color.FromBgra5551(bgra);
@@ -285,7 +284,7 @@ namespace SixLabors.ImageSharp.Formats.Tga
                                 color.FromGray8(Unsafe.As<byte, Gray8>(ref palette[bufferSpan[idx] * colorMapPixelSizeInBytes]));
                                 break;
                             case 2:
-                                // Set bit 16 to 1, to treat it as opaque for Bgra5551.
+                                // Set alpha value to 1, to treat it as opaque for Bgra5551.
                                 Bgra5551 bgra = Unsafe.As<byte, Bgra5551>(ref palette[bufferSpan[idx] * colorMapPixelSizeInBytes]);
                                 bgra.PackedValue = (ushort)(bgra.PackedValue | 0x8000);
                                 color.FromBgra5551(bgra);
@@ -449,7 +448,7 @@ namespace SixLabors.ImageSharp.Formats.Tga
                                 color.FromGray8(Unsafe.As<byte, Gray8>(ref bufferSpan[idx]));
                                 break;
                             case 2:
-                                // Set bit 16 to 1, to treat it as opaque for Bgra5551.
+                                // Set alpha value to 1, to treat it as opaque for Bgra5551.
                                 bufferSpan[idx + 1] = (byte)(bufferSpan[idx + 1] | 128);
                                 color.FromBgra5551(Unsafe.As<byte, Bgra5551>(ref bufferSpan[idx]));
                                 break;
@@ -590,7 +589,7 @@ namespace SixLabors.ImageSharp.Formats.Tga
             this.tgaMetadata = this.metadata.GetFormatMetadata(TgaFormat.Instance);
             this.tgaMetadata.BitsPerPixel = (TgaBitsPerPixel)this.fileHeader.PixelDepth;
 
-            // Bit at position 3 of the descriptor indicates, that the origin is top left instead of bottom right.
+            // Bit at position 5 of the descriptor indicates, that the origin is top left instead of bottom right.
             if ((this.fileHeader.ImageDescriptor & (1 << 5)) != 0)
             {
                 return true;

src/ImageSharp/Formats/Tga/TgaEncoderCore.cs

@@ -2,6 +2,7 @@
 // Licensed under the Apache License, Version 2.0.
 
 using System;
+using System.Buffers.Binary;
 using System.IO;
 using System.Numerics;
 using System.Runtime.CompilerServices;
@@ -29,6 +30,11 @@ namespace SixLabors.ImageSharp.Formats.Tga
         /// </summary>
         private Configuration configuration;
 
+        /// <summary>
+        /// Reusable buffer for writing data.
+        /// </summary>
+        private readonly byte[] buffer = new byte[2];
+
         /// <summary>
         /// The color depth, in number of bits per pixel.
         /// </summary>
@@ -74,7 +80,7 @@ namespace SixLabors.ImageSharp.Formats.Tga
                 imageType = this.useCompression ? TgaImageType.RleBlackAndWhite : TgaImageType.BlackAndWhite;
             }
 
-            // If compression is used, set byte 3 of the image descriptor to indicate an left top origin.
+            // If compression is used, set bit 5 of the image descriptor to indicate an left top origin.
             byte imageDescriptor = (byte)(this.useCompression ? 32 : 0);
 
             var fileHeader = new TgaFileHeader(
@@ -89,7 +95,7 @@ namespace SixLabors.ImageSharp.Formats.Tga
                 width: (short)image.Width,
                 height: (short)image.Height,
                 pixelDepth: (byte)this.bitsPerPixel.Value,
-                imageDescriptor: (byte)(this.useCompression ? 32 : 0));
+                imageDescriptor: imageDescriptor);
 
 #if NETCOREAPP2_1
             Span<byte> buffer = stackalloc byte[TgaFileHeader.Size];
@@ -174,10 +180,10 @@ namespace SixLabors.ImageSharp.Formats.Tga
                         break;
 
                     case TgaBitsPerPixel.Pixel16:
-                        // TODO: this seems to be wrong
                         var bgra5551 = new Bgra5551(color.ToVector4());
-                        stream.WriteByte((byte)(bgra5551.PackedValue & 0xFF));
-                        stream.WriteByte((byte)(bgra5551.PackedValue & 0xFF00));
+                        BinaryPrimitives.TryWriteInt16LittleEndian(this.buffer, (short)bgra5551.PackedValue);
+                        stream.WriteByte(this.buffer[0]);
+                        stream.WriteByte(this.buffer[1]);
 
                         break;