scale an image using nearest neighbor interpolation 近邻插值代码

/****************************************************************************
 * Func: scale_pnm                                                          *
 *                                                                          *
 * Desc: scale an image using nearest neighbor interpolation                *
 *                                                                          *
 * Params: buffer - pointer to image in memory                              *
 *         fileout - name of output file                                    *
 *         rows - number of rows in image                                   *
 *         cols - number of columns in image                                *
 *         x_scale - scale factor in X direction                            *
 *         y_scale - scale factor in Y direction                            *
 *         type - graphics file type (5 = PGM    6 = PPM)                   *
 ****************************************************************************/
void scale_pnm(image_ptr buffer, char *fileout, int rows, int cols,
       float x_scale, float y_scale, int type)
    {
    unsigned long x,y;          /* loop indices for columns and rows */
    unsigned long index;        /* index into line buffer */
    unsigned long source_index; /* address of source pixel in image buffer */
    unsigned char *line_buff;   /* output line buffer */
    int new_rows, new_cols;     /* values of rows and columns for new image */
    unsigned line;              /* number of pixels in one scan line */
    FILE *fp;                   /* output file pointer */
    unsigned long X_Source, Y_Source;  /* x and y address of source pixel */
    pixel_ptr color_buff;       /* pointer to a color image in memory */

    /* open new output file */
    if((fp=fopen(fileout, "wb")) == NULL)
{
printf("Unable to open %s for output\n",fileout);
exit(1);
}

    new_cols = cols * x_scale;
    new_rows = rows * y_scale;

    /* print out the portable bitmap header */
    fprintf(fp, "P%d\n%d %d\n255\n", type, new_cols, new_rows);

    if(type == 5)        /* PGM file */
line = new_cols;
    else                 /* PPM file */
{
line = new_cols * 3;
color_buff = (pixel_ptr) buffer;
}

    line_buff = (unsigned char *) malloc(line);

    for(y=0; y<new_rows; y++)
{
index = 0;
for(x=0; x<new_cols; x++)
    {
    X_Source = (unsigned long)((x / x_scale) + 0.5);
    Y_Source = (unsigned long)((y / y_scale) + 0.5);
    source_index = Y_Source * cols + X_Source;
    if(type == 5)      /* PGM */
line_buff[index++] = buffer[source_index];
    else               /* PPM */
{
line_buff[index++] = color_buff[source_index].r;
line_buff[index++] = color_buff[source_index].g;
line_buff[index++] = color_buff[source_index].b;
}
    }
fwrite(line_buff, 1, line, fp);
}
    fclose(fp);
    }