initial commit

main.go

@@ -0,0 +1,197 @@
+package main
+
+import (
+	"bytes"
+	"compress/lzw"
+	"encoding/binary"
+	"fmt"
+	"io"
+	"os"
+	"strings"
+)
+
+func readNextBytes(file *os.File) byte {
+	nextBytes := make([]byte, 1)
+	file.Read(nextBytes)
+	file.Seek(-1, os.SEEK_CUR)
+
+	return nextBytes[0]
+}
+
+func (v Palette) UnmarshalBinary(data []byte) error {
+	if len(v)*3 != len(data) {
+		return fmt.Errorf("len is not valid. required: %d, actual: %d", len(v)*3, len(data))
+	}
+	for i := 0; i < len(v); i++ {
+		v[i].Red = data[i*3]
+		v[i].Green = data[i*3+1]
+		v[i].Blue = data[i*3+2]
+	}
+	return nil
+}
+
+func main() {
+	inputFile := os.Args[1]
+
+	file, err := os.OpenFile(inputFile, os.O_RDONLY, 0x755)
+	if err != nil {
+		panic(err)
+	}
+
+	header := Header{}
+	headerData := make([]byte, 13)
+	file.Read(headerData)
+	binary.Read(bytes.NewBuffer(headerData), binary.LittleEndian, &header)
+
+	if strings.ToUpper(string(header.Signature[:])) != "GIF" {
+		panic("Not a valid gif file")
+	}
+
+	fmt.Print("Beginning extraction of gif frames\n\n")
+
+	fmt.Printf("GIF version is: %v\n", string(header.Version[:]))
+	if string(header.Version[:]) == "89a" {
+		fmt.Print("WARNING: some features are unimplemented for this version. Some images may not extract correctly\n\n")
+	}
+	fmt.Printf("GIF aspect ratio is: %v\n", header.AspectRatio)
+	fmt.Printf("GIF background color is: %v\n", header.BackgroundColor)
+	fmt.Printf("GIF height is: %v\n", header.ScreenHeight)
+	fmt.Printf("GIF width is: %v\n", header.ScreenWidth)
+
+	GlobalColorTableFlag := (header.Packed & 0x80) >> 7
+	GlobalColorTableEntries := 1 << ((header.Packed & 7) + 1)
+	GlobalColorTableSize := 3 * GlobalColorTableEntries // number of entries * 3 colors (r, g, b)
+
+	fmt.Printf("\n")
+	fmt.Printf("Global color table flag: %v\n", GlobalColorTableFlag)
+	fmt.Printf("Global color table size: %v\n", GlobalColorTableSize)
+	fmt.Printf("global color table entries: %v\n", GlobalColorTableEntries)
+
+	var palette Palette
+
+	if GlobalColorTableFlag == 0 { // no global color table, use default one (???)
+		// TODO: do we skip it. no idea if this is accurate
+		file.Seek(int64(GlobalColorTableSize), os.SEEK_CUR)
+	} else { // global color table exists, use it
+		globalColorTable := make([]byte, GlobalColorTableSize)
+		file.Read(globalColorTable)
+
+		palette = make(Palette, GlobalColorTableEntries)
+		palette.UnmarshalBinary(globalColorTable)
+	}
+
+	for i := 1; readNextBytes(file) != TRAILER; i++ {
+		nextByte := make([]byte, 1)
+		file.Read(nextByte)
+
+		fmt.Printf("next byte is: 0x%x\n", nextByte[0])
+
+		switch nextByte[0] {
+		case EXTENSION_BLOCK:
+			{
+				nextSecondByte := make([]byte, 1)
+				file.Read(nextSecondByte)
+
+				fmt.Printf("next second byte is: 0x%x\n", nextSecondByte[0])
+
+				switch nextSecondByte[0] {
+				case PLAINTEXT_BLOCK:
+					{
+						file.Seek(PLAINTEXT_BLOCK_SIZE+1, os.SEEK_CUR)
+						plaintextDataSize := make([]byte, 1)
+						file.Read(plaintextDataSize)
+						file.Seek(int64(plaintextDataSize[0]), os.SEEK_CUR)
+
+						// TODO: read plaintext data and draw it to the frame (somehow idk ???)
+
+						break
+					}
+				case GRAPHICS_CONTROL_BLOCK:
+					{
+						file.Seek(GRAPHICS_CONTROL_BLOCK_SIZE+1, os.SEEK_CUR)
+
+						// TODO: read graphics control block and do something with it ???
+
+						break
+					}
+				case APPLICATION_BLOCK:
+					{
+						file.Seek(APPLICATION_BLOCK_SIZE+1, os.SEEK_CUR)
+						applicationDataSize := make([]byte, 1)
+						file.Read(applicationDataSize)
+						file.Seek(int64(applicationDataSize[0]), os.SEEK_CUR)
+
+						// TODO: read application data and do something with it ???
+
+						break
+					}
+				case COMMENT_BLOCK:
+					{
+						commentDataSize := make([]byte, 1)
+						file.Read(commentDataSize)
+						file.Seek(int64(commentDataSize[0]), os.SEEK_CUR)
+
+						// TODO: read comment data and output it to file ???
+
+						break
+					}
+				}
+				break
+			}
+		case IMAGE_DESCRIPTOR:
+			{
+				imageDescriptor := ImageDescriptor{}
+				imageDescriptorData := make([]byte, 9)
+				file.Read(imageDescriptorData)
+				binary.Read(bytes.NewBuffer(imageDescriptorData), binary.LittleEndian, &imageDescriptor)
+
+				decompressedFrameSize := uint64(int64(imageDescriptor.Height) * int64(imageDescriptor.Width))
+
+				LocalColorTableFlag := imageDescriptor.Packed & 1
+				InterlaceFlag := (imageDescriptor.Packed & 2) >> 1
+				SortFlag := (imageDescriptor.Packed & 4) >> 2
+				LocalColorTableSize := (imageDescriptor.Packed & 0xE0) >> 5
+
+				fmt.Printf("Local Color Table Flag: %v\n", LocalColorTableFlag)
+				fmt.Printf("Interlace Flag: %v\n", InterlaceFlag)
+				fmt.Printf("Sort Flag: %v\n", SortFlag)
+				fmt.Printf("Local Color Table Size: %v\n", LocalColorTableSize)
+
+				// TODO: implement deinterlacing
+				// TODO: implement sort flag thingy ???
+				// TODO: implement local color table
+				// TODO: use color tables for correct colors on pixels
+				// TODO: write the image data to PNG
+
+				LZWCodeSize := make([]byte, 1)
+				file.Read(LZWCodeSize)
+
+				reader := lzw.NewReader(newBlockReader(file), lzw.LSB, int(LZWCodeSize[0]))
+				defer reader.Close()
+
+				decompressedFrameData := make([]byte, decompressedFrameSize)
+
+				_, err = io.ReadFull(reader, decompressedFrameData)
+				if err != nil {
+					panic(err)
+				}
+
+				outFile, err := os.OpenFile(fmt.Sprintf("%s-raw.%v", file.Name(), i), os.O_CREATE|os.O_RDWR, 0755)
+				if err != nil {
+					panic(err)
+				}
+
+				outFile.Write(decompressedFrameData)
+
+				break
+			}
+		}
+
+		// skip the terminator byte
+		file.Seek(1, os.SEEK_CUR)
+	}
+
+	fmt.Print("Extracted frames from gif successfully!\n")
+
+	file.Close()
+}