initial commit

datatypes.go

@@ -0,0 +1,112 @@
+package main
+
+const (
+	EXTENSION_BLOCK = 0x21
+
+	GRAPHICS_CONTROL_BLOCK      = 0xF9
+	GRAPHICS_CONTROL_BLOCK_SIZE = 0x04
+
+	PLAINTEXT_BLOCK      = 0x01
+	PLAINTEXT_BLOCK_SIZE = 0x0C
+
+	APPLICATION_BLOCK      = 0xFF
+	APPLICATION_BLOCK_SIZE = 0x0B
+
+	COMMENT_BLOCK    = 0xFE
+	IMAGE_DESCRIPTOR = 0x2C
+	TRAILER          = 0x3B
+)
+
+/*
+HeaderPacked {
+	0-2: 	GlobalColorTableSize
+	  3: 	ColorTableSortFlag   | Only valid under 89a, 87a always sets it to 0
+	4-6:	ColorResolution
+	  7:	GlobalColorTableFlag
+}
+*/
+
+type Header struct {
+	Signature [3]byte // "GIF"
+	Version   [3]byte // "87a" or "89a"
+
+	// Logical Screen Descriptor
+	ScreenWidth     uint16
+	ScreenHeight    uint16
+	Packed          byte
+	BackgroundColor byte // unused if GlobalColorTableFlag is unset
+	AspectRatio     byte
+}
+
+// [OPTIONAL]
+// Comes right after the logical screen descriptor
+// Size of color table is always a power of 2, with a max of 256 entries in the table
+// ColorTableEntries = 1 << ((Packed & 7) + 1)
+// ColorTableSize = 3 * (1 << ((Packed & 7) + 1))
+type RGB struct {
+	Red   byte
+	Green byte
+	Blue  byte
+}
+
+type Palette []RGB
+
+/*
+ImageDescriptorPacked {
+	0:   LocalColorTableFlag | this flag is set (1) if the image contains a local color table
+	1:   InterlaceFlag       | this flag is set (1) if the image is interlaced
+	2:   SortFlag            | this flag is set (1) if the color table is sorted by importance (frequency of occurrence). only available on 89a
+	3-4: Reserved
+	5-7: LocalColorTableEntrySize
+}
+*/
+
+type ImageDescriptor struct {
+	Left   int16  // X position of image
+	Top    int16  // Y position of image
+	Width  uint16 // width of image in pixels
+	Height uint16 // height of image in pixels
+	Packed byte   // image and color table data information
+}
+
+/*
+GraphicsControlPacked {
+	0:   TransparentColorFlag
+	1:   UserInputFlag
+	2-4: DisposalMethod
+	5-7: Reserved
+}
+*/
+
+/********************/
+/* EXTENSION BLOCKS */
+/********************/
+
+// Only available on 89a and comes after the global color table and before the 1st image
+type GraphicsControlBlock struct {
+	Packed                byte  // method of graphics disposal to use
+	DelayTime             int16 // delay to wait
+	TransparentColorIndex byte  // transparent color index
+}
+
+type PlainTextBlock struct {
+	TextGridLeft     int16 // X position of text grid in pixels
+	TextGridTop      int16 // Y position of the text grid in pixels
+	TextGridWidth    int16 // width of text grid in pixels
+	TextGridHeight   int16 // height of text grid in pixels
+	CellWidth        byte  // width of grid cell in pixels
+	CellHeight       byte  // height of grid cell in pixels
+	TextFgColorIndex byte  // text foreground color index value
+	TextBgColorIndex byte  // text background color index value
+	PlainTextData    *byte // the plaintext data
+}
+
+type ApplicationExtensionBlock struct {
+	Identifier      [8]byte // application identifier
+	AuthentCode     [3]byte // application authentication code
+	ApplicationData *byte   // application data
+}
+
+type CommentExtensionBlock struct {
+	CommentData *byte // comment data
+}