Code archives/Algorithms/RLE compression

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RLE compression by Zenith
(Posted 24 years ago)
Wow, am I the first to put RLE compression on here?
Pretty simple to use, 2 small functions. :)

This kind of compression is really good for cleaning up files that repeat the same character over and over again alot.

Using it for my hardware projects.. 
Function RLE_compress(in_file$,out_file$)
	Local temp$
	ifile = ReadFile(in_file)
	ofile = WriteFile(out_file)
	oldbyte = -1
	While Not Eof(ifile)
	
		newbyte = ReadByte(ifile)
		
		If newbyte = oldbyte				; another byte! Lets add it to the list :)
			If rcount = 255					; we have TOO many in the list :)
				WriteByte(ofile,rcount)
				WriteByte(ofile,newbyte)
				rcount = 0
			ElseIf rcount = 0 And Len(temp)>0
				WriteByte(ofile,0)
				WriteByte(ofile,Len(temp))
				For i=1 To Len(temp)
					WriteByte(ofile,Asc(Mid(temp,i,1)))
				Next
				temp=""
				rcount = rcount + 1
			Else
				rcount = rcount + 1
			EndIf
		Else								; new byte type! Lets write off the old byte list	
			If oldbyte>-1
				If rcount>0
					WriteByte(ofile,rcount)
					WriteByte(ofile,oldbyte)
				Else
					If Len(temp)=255
						WriteByte(ofile,0)
						WriteByte(ofile,Len(temp))
						For i=1 To Len(temp)
							WriteByte(ofile,Asc(Mid(temp,i,1)))
						Next
						temp = Chr(oldbyte)
					Else
						temp = temp + Chr(oldbyte)
					EndIf
				EndIf
			EndIf
			oldbyte = newbyte
			rcount = 0
		EndIf
		
	Wend
	
	If rcount>0
		WriteByte(ofile,rcount)
		WriteByte(ofile,oldbyte)
	EndIf
	
	CloseFile(ifile)
	CloseFile(ofile)
End Function

Function RLE_uncompress(in_file$,out_file$)
	ifile = ReadFile(in_file)
	ofile = WriteFile(out_file)
	While Not Eof(ifile)
		rcount = ReadByte(ifile)
		If rcount = 0
			length = ReadByte(ifile)
			For i=1 To length
				WriteByte ofile,ReadByte(ifile)
			Next
		Else
			arg	   = ReadByte(ifile)
			For i=0 To rcount
				WriteByte ofile,arg
			Next
		EndIf
	Wend
	CloseFile(ifile)
	CloseFile(ofile)
End Function

Comments

Cygnus
(Posted 23 years ago)
excelent. Il be number two, Im no expert in file compression but i was chuffed to see this work quite nicely!:


Function simplecompress(name$,outputname$)
fs=WriteFile(outputname$)
fs2=ReadFile(name$)
bt=0
Repeat
count=0
Repeat
obt=bt
bt=ReadByte(fs2)
count=count+1
Until (bt<>obt And obt<>-1) Or count>254 Or Eof(fs2)
WriteByte fs,count
WriteByte fs,obt
Until Eof(fs2)
CloseFile fs2
CloseFile fs
DeleteFile name$
End Function


Function simpledecompress(name$,tempname$)
fs=WriteFile(tempname$)
fs2=ReadFile(name$)
dn=0
Repeat
count=ReadByte(fs2)
Char=ReadByte(fs2)
If dn>0 Then For z=1 To count:WriteByte fs,char:Next
dn=1
Until Eof(fs2)
CloseFile fs2
CloseFile fs
End Function


Problem being with mine, a file with every char different ends up twice the size....


..and....
Anyone tell me how to put that into a code box???

puki
(Posted 23 years ago)
"Cygnus" the various codes are in here:

http://www.blitzbasic.com/faq/faq_entry.php?id=2

TomToad
(Posted 23 years ago)
RLE is best used with images that have huge blocks of the same color, such as cartoon characters or illustrations. Terrible for photographs or highly detailed images. If your interested in learning more about compression, a google search brings up a lot of relavent info. Try looking at huffman compression or LZH compression, among others.

If there's a specific reason why you want to use RLE, a modification will help to improve it. Not only have repeat next byte n times, but also write next n bytes as is. The format would look something like this:

read countbyte.
if countbyte is 0 - 127, then write next byte countbyte+1 times
if countbyte is 128-255, then write the next countbyte-127 bytes as is.

So
1 2 3 3 3 3 3 4 3 4 5 4 4 4 4 4 4 4
would encode as
129 1 2 4 3 131 4 3 4 5 6 4

Zenith
(Posted 22 years ago)
Yeah, that's what I have. :)

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