Problem using a compression library

[abu]

I load a bitmap and compress it bitmap data using LZ-77 algorithm.


I want to store the current address of pointer of file stream f1,f2 in unsigned char *in,out;
I tried ftell() , fseek() functions but it return an long and cannot convert to unsigned char* in.
Help me!!!

[swoopy]

> int outsize=LZ_Compress(in,out,size);

You haven't allocated any memory for in and out at this point. Plus out is declared as an unsigned char.

[abu]

So my question is how to get the 2 pointers
unsigned char* in,out
to point to the current address of the pointer of the FILE* f1,f2.

[quzah]

You don't. It doesn't work that way. Your functions don't work with file streams. How about reading up on your functions to see how they're supposed to be used? (Since it's obvious that you didn't write either of the two functions in bold, because if you had, you'd know how they work, so you wouldn't be asking how to use them.)

In other words, RTFM.

Quzah.

[abu]

Sorry if my question is not clear.I tried to use the library "lz.c" in my program.But it still doesn't work correctly.Anyone can help me??

#include "bitmap.h"
#include<stdio.h>
#include<iostream.h>

#include "lz.c"


int LZ_Compress( unsigned char *in, unsigned char *out,
                 unsigned int insize );
void LZ_Uncompress( unsigned char *in, unsigned char *out,
                    unsigned int insize );

int main()
{

	unsigned char *out;
	//load bitmap file
	Fileheader bmfh;
	Infoheader bmih;	
	RGBQUAD Palette[256];
	unsigned char* Data;
	FILE *f1;
	FILE *f2;
	f1=fopen("test.bmp","rb");
	f2=fopen("LZ.77","wb");
		
		fread(&bmfh,sizeof(Fileheader),1,f1);
		fread(&bmih,sizeof(Infoheader),1,f1);
	
	for(int i=0;i<256;i++)
	{
		fread(&Palette[i],sizeof(RGBQUAD),1,f1);
	}
	
	//read the actual bitmap data 
	fseek(f1,bmfh.OffBits,SEEK_SET);
	long size=(((bmih.Width * (bmih.Bitperpixel / 8)) + 3) & (~3))*bmih.Height;
	
	Data=new unsigned char[size];
	fread((unsigned char*)Data,size,1,f1);
			
	fwrite(&bmfh,sizeof(Fileheader),1,f2);
	fwrite(&bmih,sizeof(Infoheader),1,f2);
	for(i=0;i<256;i++)
	{
		fwrite(&Palette[i],sizeof(RGBQUAD),1,f2);
	}

	out=new unsigned char[size];
                            //pointer to output buffer
                // begin LZ77 compression
	int insize=4096;
	int outsize=LZ_Compress(Data,out,insize);
	fwrite(&out,outsize,1,f2);
	fclose(f1);
	fclose(f2);
}

Here is this guide to used the libray( file included "lz.c")

Syntax: outsize = LZ_Compress(in,out,insize)
outsize - Size of output buffer after compression
in - Pointer to the input buffer (uncompressed data)
out - Pointer to the output buffer (compressed data)
insize - Size of input buffer

The output buffer must be able to hold (insize*257/256 + 1) bytes.

[Salem]

Moved to C++ forum

> int insize=4096;
How does this relate to (insize*257/256 + 1) bytes.

> out=new unsigned char[size];
Besides, you only allocate 'size' bytes, not 4096 (unless you're that lucky!)
So lying to a function about how much memory it has is a bad idea

> fwrite(&out,outsize,1,f2);
Should be
fwrite(out,outsize,1,f2);

out is already a pointer, you don't need another &

[abu]

> int insize=4096;
How does this relate to (insize*257/256 + 1) bytes.

insize is size of input buffer so I think it's a user defined const.

> out=new unsigned char[size];
Besides, you only allocate 'size' bytes, not 4096 (unless you're that lucky!)
So lying to a function about how much memory it has is a bad idea

size variable is the size of input file.
Because this is a compress program so I allocate 'size' bytes for output buffer so it's remaining and I only fwrite "outsize" byte.

> fwrite(&out,outsize,1,f2);
Should be
fwrite(out,outsize,1,f2);

out is already a pointer, you don't need another &

[/quote]
Thanks , I'm trying to decode the output file...hope I encoded the bitmap file correct.

[abu]

> int insize=4096;
How does this relate to (insize*257/256 + 1) bytes.

insize is size of input buffer so I think it's a user defined const.

> out=new unsigned char[size];
Besides, you only allocate 'size' bytes, not 4096 (unless you're that lucky!)
So lying to a function about how much memory it has is a bad idea

size variable is the size of input file.
Because this is a compress program so I allocate 'size' bytes for output buffer so it's remaining and I only fwrite "outsize" byte.

> fwrite(&out,outsize,1,f2);
Should be
fwrite(out,outsize,1,f2);

out is already a pointer, you don't need another &

[/quote]
Thanks , I'm trying to decode the output file...hope I encoded the bitmap file correct.

[Salem]

> long size=(((bmih.Width * (bmih.Bitperpixel / 8)) + 3) & (~3))*bmih.Height;
Doesn't look const to me - it varies with every single file you try and read in.

Take a snapshot of the screen say (1024*768*32) and your code is in big trouble.

> Because this is a compress program so I allocate 'size' bytes for output buffer
Look at the calculation - the output buffer has to be bigger than the input buffer - temporarily at least.
If the file turns out to be not compressable at all, then the file will expand by a very small amount - namely by 1 extra byte for every 256 bytes of input.