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I have image in pgm

enter image description here

after using this function:

void convertWithDithering(array_type& pixelgray)
{

    int oldpixel;
    int newpixel;
    int quant_error;

    for (int y = 0; y< HEIGHT-1; y++){
        for (int x = 1; x<WIDTH-1; x++){
                oldpixel = pixelgray[x][y];
                 newpixel  = (oldpixel > 128) ? 0 : 1;
                 pixelgray[x][y] = newpixel;
                  quant_error  = oldpixel - newpixel;
                 pixelgray[x+1][y]  =  pixelgray[x+1][y] + 7/16 * quant_error;
                pixelgray[x-1][y+1]  = pixelgray[x-1][y+1] + 3/16 * quant_error;
                pixelgray[x  ][y+1]=pixelgray[x  ][y+1]+  5/16 * quant_error;
                 pixelgray[x+1][y+1] = pixelgray[x+1][y+1]+ 1/16 * quant_error;
        }
    }

}

i have this

enter image description here

I want to get the same image only in black white colors

Матвей Козин
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    First thing to check when your image smears is the stride. Is there padding at the end of the line that you don't handle correctly? Are you stepping over pixels properly (check your multipliers, make sure `x` and `y` are in the right order throughout, make sure you're taking pixel size into account, etc). – ssube Nov 19 '13 at 15:06

2 Answers2

2

Last time I had a simililar smeering with a PGM file, it was because I saved data in a file opened by fopen(filename,"w");

The file had a lot of \r\n line endings (os: windows), whereas it needed only \n. Maybe your issue is something like that. Save the file in binary format, with

fopen(filename,"wb");

Edit: Asides from the smeering, your implementation of Floyd–Steinberg dithering is incorrect.

First, your error propagation should be XXX * quant_error /16 instead of XXX/16 * quant_error (which will always be equal to 0).

Then, you are mixing up the two color spaces (0/1 and 0->255). A correct way to handle it is to always use the 0->255 space by changing to test line to

newpixel = (oldpixel > 128) ? 255 : 0; (note that the order 255 : 0 is important, if you let 0 : 255, the algorithm won't work)

At the end of the function, your array will be full or 0 or 255. If you want, you can iterate one more time to convert it to 0-1, but I think it is easier to do it once you record your pbm file.

biskitt
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0

It looks like the conversion to PBM outside of this function is doing something wrong. It looks like it is converting one PGM pixel to several PBM pixels, which results in this 'smeering' effect. Just a wild guess tho. The function itself looks okay to me, apart from one little thing: I think due to you using int for everything, all your 5/16 * quant_error will be zero. Rather use floats or doubles and make it 5.0/16.0.

influjensbahr
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