imagemagick : saving MagickExportImagePixels's output blob to gray image file? - imagemagick

Question

zbar engine sample source(zbarimg.c) shows the following.
https://github.com/ZBar/ZBar/blob/master/zbarimg/zbarimg.c
size_t bloblen = width * height;
unsigned char *blobdata = malloc(bloblen);
MagickExportImagePixels(images, 0, 0, width, height, "I", CharPixel, blobdata);
I'd like to see the blobdata.
How can I save the blobdata to file?
I made save_imgdata function to save blobdata.
int save_imgdata(char* imgf, int width, int height, char *raw)
{
PixelWand *p_wand = NULL;
PixelIterator *iterator = NULL;
PixelWand **pixels = NULL;
unsigned long x, y;
char hex[128];
//MagickWandGenesis();
p_wand = NewPixelWand();
PixelSetColor(p_wand, "gray");
//PixelSetColor(p_wand, "white");
MagickWand *m_wand = NewMagickWand(); //CORE_RL_wand_.lib;
MagickSetImageDepth(m_wand, 8);
MagickNewImage(m_wand, width, height, p_wand);
// Get a new pixel iterator
iterator = NewPixelIterator(m_wand);
for (y = 0; y<height; y++) {
// Get the next row of the image as an array of PixelWands
pixels = PixelGetNextIteratorRow(iterator, &x);
// Set the row of wands to a simple gray scale gradient
for (x = 0; x<width; x++) {
sprintf(hex, "#%02x", *raw++);
//sprintf(hex, "#%02%x02%x02x", *raw, *raw, *raw); raw++;
PixelSetColor(pixels[x], hex);
}
// Sync writes the pixels back to the m_wand
PixelSyncIterator(iterator);
}
MagickWriteImage(m_wand, imgf);
DestroyMagickWand(m_wand);
return 0;
}
The call of save_imgdata("imgw.bmp", width, height, blobdata)
save 24bpp image.
What's wrong of save_imgdata?
I want it saves 8bpp gray image file.

Answer 1

Don't bother iterating and building dynamic color/pixel values -- It's slow and resource intensive. If the data came from an export method, than use the import method to restore.
int save_imgdata(char* imgf, int width, int height, void * raw)
{
MagickWand * wand;
PixelWand * bgcolor;
bgcolor = NewPixelWand();
PixelSetColor(bgcolor, "WHITE");
wand = NewMagickWand();
MagickNewImage(wand, width, height, bgcolor);
bgcolor = DestroyPixelWand(bgcolor);
MagickSetImageDepth(wand, 8);
MagickSetImageColorspace(wand, GRAYColorspace);
MagickImportImagePixels(wand, 0, 0, width, height, "I", CharPixel, raw);
MagickQuantizeImage(wand,
256, // Reduce to 8bpp
GRAYColorspace, // Match colorspace
0, // Calculate optimal tree depth
MagickTrue, // Use dither ? This changes in IM-7
MagickFalse); // Messure Error
MagickWriteImage(wand, imgf);
wand = DestroyMagickWand(wand);
return 0;
}