public GifHeaderDirectory Extract([NotNull] SequentialReader reader)
{
var directory = new GifHeaderDirectory();
// FILE HEADER
//
// 3 - signature: "GIF"
// 3 - version: either "87a" or "89a"
//
// LOGICAL SCREEN DESCRIPTOR
//
// 2 - pixel width
// 2 - pixel height
// 1 - screen and color map information flags (0 is LSB)
// 0-2 Size of the global color table
// 3 Color table sort flag (89a only)
// 4-6 Color resolution
// 7 Global color table flag
// 1 - background color index
// 1 - pixel aspect ratio
reader.IsMotorolaByteOrder = false;
try
{
var signature = reader.GetString(3);
if (signature != "GIF")
{
directory.AddError("Invalid GIF file signature");
return directory;
}
var version = reader.GetString(3);
if (version != Gif87AVersionIdentifier && version != Gif89AVersionIdentifier)
{
directory.AddError("Unexpected GIF version");
return directory;
}
directory.Set(GifHeaderDirectory.TagGifFormatVersion, version);
directory.Set(GifHeaderDirectory.TagImageWidth, reader.GetUInt16());
directory.Set(GifHeaderDirectory.TagImageHeight, reader.GetUInt16());
var flags = reader.GetByte();
// First three bits = (BPP - 1)
var colorTableSize = 1 << ((flags & 7) + 1);
directory.Set(GifHeaderDirectory.TagColorTableSize, colorTableSize);
if (version == Gif89AVersionIdentifier)
{
var isColorTableSorted = (flags & 8) != 0;
directory.Set(GifHeaderDirectory.TagIsColorTableSorted, isColorTableSorted);
}
var bitsPerPixel = ((flags & 0x70) >> 4) + 1;
directory.Set(GifHeaderDirectory.TagBitsPerPixel, bitsPerPixel);
var hasGlobalColorTable = (flags & 0xf) != 0;
directory.Set(GifHeaderDirectory.TagHasGlobalColorTable, hasGlobalColorTable);
directory.Set(GifHeaderDirectory.TagBackgroundColorIndex, reader.GetByte());
int aspectRatioByte = reader.GetByte();
if (aspectRatioByte != 0)
{
var pixelAspectRatio = (float)((aspectRatioByte + 15d) / 64d);
directory.Set(GifHeaderDirectory.TagPixelAspectRatio, pixelAspectRatio);
}
}
catch (IOException)
{
directory.AddError("Unable to read BMP header");
}
return directory;
}
/// <summary>Reads the fixed-position GIF header.</summary>
private static GifHeaderDirectory ReadGifHeader(SequentialReader reader)
{
// FILE HEADER
//
// 3 - signature: "GIF"
// 3 - version: either "87a" or "89a"
//
// LOGICAL SCREEN DESCRIPTOR
//
// 2 - pixel width
// 2 - pixel height
// 1 - screen and color map information flags (0 is LSB)
// 0-2 Size of the global color table
// 3 Color table sort flag (89a only)
// 4-6 Color resolution
// 7 Global color table flag
// 1 - background color index
// 1 - pixel aspect ratio
var headerDirectory = new GifHeaderDirectory();
var signature = reader.GetString(3, Encoding.UTF8);
if (signature != "GIF")
{
headerDirectory.AddError("Invalid GIF file signature");
return headerDirectory;
}
var version = reader.GetString(3, Encoding.UTF8);
if (version != Gif87AVersionIdentifier && version != Gif89AVersionIdentifier)
{
headerDirectory.AddError($"Unexpected GIF version \"{version}\"");
return headerDirectory;
}
headerDirectory.Set(GifHeaderDirectory.TagGifFormatVersion, version);
// LOGICAL SCREEN DESCRIPTOR
headerDirectory.Set(GifHeaderDirectory.TagImageWidth, reader.GetUInt16());
headerDirectory.Set(GifHeaderDirectory.TagImageHeight, reader.GetUInt16());
var flags = reader.GetByte();
// First three bits = (BPP - 1)
var colorTableSize = 1 << ((flags & 7) + 1);
var bitsPerPixel = ((flags & 0x70) >> 4) + 1;
var hasGlobalColorTable = (flags >> 7) != 0;
headerDirectory.Set(GifHeaderDirectory.TagColorTableSize, colorTableSize);
if (version == Gif89AVersionIdentifier)
{
var isColorTableSorted = (flags & 8) != 0;
headerDirectory.Set(GifHeaderDirectory.TagIsColorTableSorted, isColorTableSorted);
}
headerDirectory.Set(GifHeaderDirectory.TagBitsPerPixel, bitsPerPixel);
headerDirectory.Set(GifHeaderDirectory.TagHasGlobalColorTable, hasGlobalColorTable);
headerDirectory.Set(GifHeaderDirectory.TagBackgroundColorIndex, reader.GetByte());
int aspectRatioByte = reader.GetByte();
if (aspectRatioByte != 0)
{
var pixelAspectRatio = (float)((aspectRatioByte + 15d) / 64d);
headerDirectory.Set(GifHeaderDirectory.TagPixelAspectRatio, pixelAspectRatio);
}
return headerDirectory;
}
public GifHeaderDirectory Extract([NotNull] SequentialReader reader)
{
var directory = new GifHeaderDirectory();
// FILE HEADER
//
// 3 - signature: "GIF"
// 3 - version: either "87a" or "89a"
//
// LOGICAL SCREEN DESCRIPTOR
//
// 2 - pixel width
// 2 - pixel height
// 1 - screen and color map information flags (0 is LSB)
// 0-2 Size of the global color table
// 3 Color table sort flag (89a only)
// 4-6 Color resolution
// 7 Global color table flag
// 1 - background color index
// 1 - pixel aspect ratio
reader.IsMotorolaByteOrder = false;
try
{
var signature = reader.GetString(3);
if (signature != "GIF")
{
directory.AddError("Invalid GIF file signature");
return(directory);
}
var version = reader.GetString(3);
if (version != Gif87AVersionIdentifier && version != Gif89AVersionIdentifier)
{
directory.AddError("Unexpected GIF version");
return(directory);
}
directory.Set(GifHeaderDirectory.TagGifFormatVersion, version);
directory.Set(GifHeaderDirectory.TagImageWidth, reader.GetUInt16());
directory.Set(GifHeaderDirectory.TagImageHeight, reader.GetUInt16());
var flags = reader.GetByte();
// First three bits = (BPP - 1)
var colorTableSize = 1 << ((flags & 7) + 1);
directory.Set(GifHeaderDirectory.TagColorTableSize, colorTableSize);
if (version == Gif89AVersionIdentifier)
{
var isColorTableSorted = (flags & 8) != 0;
directory.Set(GifHeaderDirectory.TagIsColorTableSorted, isColorTableSorted);
}
var bitsPerPixel = ((flags & 0x70) >> 4) + 1;
directory.Set(GifHeaderDirectory.TagBitsPerPixel, bitsPerPixel);
var hasGlobalColorTable = (flags & 0xf) != 0;
directory.Set(GifHeaderDirectory.TagHasGlobalColorTable, hasGlobalColorTable);
directory.Set(GifHeaderDirectory.TagTransparentColorIndex, reader.GetByte());
int aspectRatioByte = reader.GetByte();
if (aspectRatioByte != 0)
{
var pixelAspectRatio = (float)((aspectRatioByte + 15d) / 64d);
directory.Set(GifHeaderDirectory.TagPixelAspectRatio, pixelAspectRatio);
}
}
catch (IOException)
{
directory.AddError("Unable to read BMP header");
}
return(directory);
}