Thread: Good Image Library

In what way could you not get that library to work in Dev-C++? Could you not get a program to compile or link?

That's a C++ library, whilst this is the C forum -- what language are you actually using?

Hi, I am looking to write my own digital watermarking program.

I don't know what's involved in doing this, but it sounds complicated . . . why are you doing this? Do you want to use an existing library?

Why not use something simple AND written in C like libgd ?

I am looking to write my own digital watermarking program. So i need a library that can extract the RGB values from an image into a matrix form, so then I can do some kind of transform on the martix, change a few values and then transform back.

You should be able to do this with nothing more than the image file load/save functions.

To extract the RGB colour components you can either read the image data byte at a time or use bit shifting to extract colour components. But I doubt you would even need that.

Say if your watermark was to slightly lighten each pixel its applied to by 1 in r, g, and b. you could 'and' each pixel on your image with a bitmask of the watermark. For each pixel to be modified you could then 'or' the existing colour with itself plus 0x010101 to increment r,g, and b in one go. To extract a watermark its just a matter of subtracting the original from a watermarked image.

Just some ideas for you.

Okay, i realised that post I made above was a bit wrong. Anyway I knocked up a quick working watermark adder in Basic and had a go at converting it to C. I havent tested this yet, but I think it ought to work. As long as I'm not doing bad stuff with the memory (as I usually do).

Code:

#define Uint8  unsigned char
#define Uint16 unsigned short
#define Uint32 unsigned int

#define COLOUR_MARK 0x00FFFFFF
#define COLOUR_HILIGHT 0x00FF0000

#define ERROR_DRAW_OUT_OF_BOUNDS 1
#define ERROR_UNEQUAL_DIMENSIONS 2

struct image
{
    Uint16 w, h;
    Uint32 *pixel;
}typedef struct image image;

/* 
   This function draws a watermark to an image
   x_pos / y_pos = start point on image to draw the watermark
   *pic = pointer to image
   *mark = pointer to watermark image 
*/
int AddWatermark(Uint32 *pic, Uint32 *mark, Uint16 x_pos, Uint16 y_pos)
{
    if(x_pos + mark.w > pic.w) return ERROR_DRAW_OUT_OF_BOUNDS;
    if(y_pos + mark.h > pic.h) return ERROR_DRAW_OUT_OF_BOUNDS;

    Uint8 r, g, b;       
    Uint16 x, y;
    Uint16 scanline = pic.w*4;
    Uint32 *m_ptr = mark.pixel;
    Uint32 *p_ptr = pic.pixel+(y_pos*scanline)+(x_pos*4); 
    
    for(y=0; y<mark.h; y++)
    {
        for(x=0; x<mark.w; x++)
        {
            if(*m_ptr == COLOUR_MARK)
            {
                r = (*p_ptr >> 16) & 255;   //Get colour elements
                g = (*p_ptr >> 8) & 255;
                b = *p_ptr & 255;
                if(r < 255) r+=1;           //Add watermark to elements
                if(g < 255) g+=1;
                if(b < 255) b+=1;
                *ptr = (r<<16)+(g<<8)+b;    //Write data back to pixel
            }
            p_ptr+=4;
            m_ptr+=4;            
        }
        p_ptr += scanline;
    }  
    return 0;
}

/* This function hilights pixels on a watermarked image that don't match the original
   *origin = handle for unaltered image
   *marked = handle for watermarked image 
*/
int GetWatermark(Uint32 *origin, *marked)
{
    if(origin.w != marked.w) return ERROR_UNEQUAL_DIMENSIONS;
    if(origin.h != marked.h) return ERROR_UNEQUAL_DIMENSIONS;   

    Uint32 *o_ptr = origin.pixel;
    Uint32 *m_ptr = marked.pixel;
    Uint16 x, y;
    for(y=0; y<origin.h; y++)
        for(x=0; x<origin.w; x++, o_ptr+=4, m_ptr+=4)
            if(! (*m_ptr - *o_ptr)) //Hilight pixels that don't match the original
                *m_ptr = COLOUR_HILIGHT; 
}

Also something I was wondering was if someone were to hide text in watermarks, that would pretty much be uncrackable to anyone that dident have the original image right? Seems like it could make some kickass encryption

It's called steganography