/// <summary>
/// Parses a OS/2 version 2 bitmap header (64 bytes). Only the first 40 bytes are parsed which are
/// very similar to the Bitmap v3 header. The other 24 bytes are ignored, but they do not hold any
/// useful information for decoding the image.
/// </summary>
/// <param name="data">The data to parse.</param>
/// <returns>The parsed header.</returns>
/// <seealso href="https://www.fileformat.info/format/os2bmp/egff.htm"/>
public static BmpInfoHeader ParseOs2Version2(ReadOnlySpan <byte> data)
{
var infoHeader = new BmpInfoHeader(
headerSize: BinaryPrimitives.ReadInt32LittleEndian(data.Slice(0, 4)),
width: BinaryPrimitives.ReadInt32LittleEndian(data.Slice(4, 4)),
height: BinaryPrimitives.ReadInt32LittleEndian(data.Slice(8, 4)),
planes: BinaryPrimitives.ReadInt16LittleEndian(data.Slice(12, 2)),
bitsPerPixel: BinaryPrimitives.ReadInt16LittleEndian(data.Slice(14, 2)));
int compression = BinaryPrimitives.ReadInt32LittleEndian(data.Slice(16, 4));
// The compression value in OS/2 bitmap has a different meaning than in windows bitmaps.
// Map the OS/2 value to the windows values.
switch (compression)
{
case 0:
infoHeader.Compression = BmpCompression.RGB;
break;
case 1:
infoHeader.Compression = BmpCompression.RLE8;
break;
case 2:
infoHeader.Compression = BmpCompression.RLE4;
break;
case 4:
infoHeader.Compression = BmpCompression.RLE24;
break;
default:
// Compression type 3 (1DHuffman) is not supported.
BmpThrowHelper.ThrowImageFormatException("Compression type is not supported. ImageSharp only supports uncompressed, RLE4, RLE8 and RLE24.");
break;
}
infoHeader.ImageSize = BinaryPrimitives.ReadInt32LittleEndian(data.Slice(20, 4));
infoHeader.XPelsPerMeter = BinaryPrimitives.ReadInt32LittleEndian(data.Slice(24, 4));
infoHeader.YPelsPerMeter = BinaryPrimitives.ReadInt32LittleEndian(data.Slice(28, 4));
infoHeader.ClrUsed = BinaryPrimitives.ReadInt32LittleEndian(data.Slice(32, 4));
infoHeader.ClrImportant = BinaryPrimitives.ReadInt32LittleEndian(data.Slice(36, 4));
// The following 24 bytes of the header are omitted.
return(infoHeader);
}
/// <summary>
/// Produce uncompressed bitmap data from a RLE8 stream.
/// </summary>
/// <remarks>
/// RLE8 is a 2-byte run-length encoding.
/// <br/>If first byte is 0, the second byte may have special meaning.
/// <br/>Otherwise, the first byte is the length of the run and second byte is the color for the run.
/// </remarks>
/// <param name="w">The width of the bitmap.</param>
/// <param name="buffer">Buffer for uncompressed data.</param>
private void UncompressRle8(int w, Span <byte> buffer)
{
#if NETCOREAPP2_1
Span <byte> cmd = stackalloc byte[2];
#else
byte[] cmd = new byte[2];
#endif
int count = 0;
while (count < buffer.Length)
{
if (this.stream.Read(cmd, 0, cmd.Length) != 2)
{
BmpThrowHelper.ThrowImageFormatException("Failed to read 2 bytes from stream while uncompressing RLE8 bitmap.");
}
if (cmd[0] == RleCommand)
{
switch (cmd[1])
{
case RleEndOfBitmap:
return;
case RleEndOfLine:
int extra = count % w;
if (extra > 0)
{
count += w - extra;
}
break;
case RleDelta:
int dx = this.stream.ReadByte();
int dy = this.stream.ReadByte();
count += (w * dy) + dx;
break;
default:
// If the second byte > 2, we are in 'absolute mode'
// Take this number of bytes from the stream as uncompressed data
int length = cmd[1];
byte[] run = new byte[length];
this.stream.Read(run, 0, run.Length);
run.AsSpan().CopyTo(buffer.Slice(count));
count += run.Length;
// Absolute mode data is aligned to two-byte word-boundary
int padding = length & 1;
this.stream.Skip(padding);
break;
}
}
else
{
int max = count + cmd[0]; // as we start at the current count in the following loop, max is count + cmd[0]
byte cmd1 = cmd[1]; // store the value to avoid the repeated indexer access inside the loop
for (; count < max; count++)
{
buffer[count] = cmd1;
}
}
}
}
/// <summary>
/// Produce uncompressed bitmap data from a RLE4 stream.
/// </summary>
/// <remarks>
/// RLE4 is a 2-byte run-length encoding.
/// <br/>If first byte is 0, the second byte may have special meaning.
/// <br/>Otherwise, the first byte is the length of the run and second byte contains two color indexes.
/// </remarks>
/// <param name="w">The width of the bitmap.</param>
/// <param name="buffer">Buffer for uncompressed data.</param>
private void UncompressRle4(int w, Span <byte> buffer)
{
#if NETCOREAPP2_1
Span <byte> cmd = stackalloc byte[2];
#else
byte[] cmd = new byte[2];
#endif
int count = 0;
while (count < buffer.Length)
{
if (this.stream.Read(cmd, 0, cmd.Length) != 2)
{
BmpThrowHelper.ThrowImageFormatException("Failed to read 2 bytes from the stream while uncompressing RLE4 bitmap.");
}
if (cmd[0] == RleCommand)
{
switch (cmd[1])
{
case RleEndOfBitmap:
return;
case RleEndOfLine:
int extra = count % w;
if (extra > 0)
{
count += w - extra;
}
break;
case RleDelta:
int dx = this.stream.ReadByte();
int dy = this.stream.ReadByte();
count += (w * dy) + dx;
break;
default:
// If the second byte > 2, we are in 'absolute mode'.
// The second byte contains the number of color indexes that follow.
int max = cmd[1];
int bytesToRead = (max + 1) / 2;
byte[] run = new byte[bytesToRead];
this.stream.Read(run, 0, run.Length);
int idx = 0;
for (int i = 0; i < max; i++)
{
byte twoPixels = run[idx];
if (i % 2 == 0)
{
byte leftPixel = (byte)((twoPixels >> 4) & 0xF);
buffer[count++] = leftPixel;
}
else
{
byte rightPixel = (byte)(twoPixels & 0xF);
buffer[count++] = rightPixel;
idx++;
}
}
// Absolute mode data is aligned to two-byte word-boundary
int padding = bytesToRead & 1;
this.stream.Skip(padding);
break;
}
}
else
{
int max = cmd[0];
// The second byte contains two color indexes, one in its high-order 4 bits and one in its low-order 4 bits.
byte twoPixels = cmd[1];
byte rightPixel = (byte)(twoPixels & 0xF);
byte leftPixel = (byte)((twoPixels >> 4) & 0xF);
for (int idx = 0; idx < max; idx++)
{
if (idx % 2 == 0)
{
buffer[count] = leftPixel;
}
else
{
buffer[count] = rightPixel;
}
count++;
}
}
}
}
