To convert 32 bpp to 1 bpp, extract RGB and convert it into Y (luminance) and use some threshold to convert to 1 bpp.
Number of samples and bits per pixel should be 1.
题
My android app uses an external lib that makes some image treatments. The final output of the treatment chain is a monochrome bitmap but saved has a color bitmap (32bpp).
The image has to be uploaded to a cloud blob, so for bandwidth concerns, i'd like to convert it to 1bpp G4 compression TIFF. I successfully integrated libTIFF in my app via JNI and now i'm writing the conversion routine in C. I'm a little stuck here.
I managed to produce a 32 BPP TIFF, but impossible to reduce to 1bpp, the output image is always unreadable. Did someone succeded to do similar task ?
More speciffically :
Many thanks !
UPDATE : The code produced with the precious help of Mohit Jain
int ConvertMonochrome32BppBitmapTo1BppTiff(char* bitmap, int height, int width, int resx, int resy, char const *tifffilename)
{
TIFF *tiff;
if ((tiff = TIFFOpen(tifffilename, "w")) == NULL)
{
return TC_ERROR_OPEN_FAILED;
}
// TIFF Settings
TIFFSetField(tiff, TIFFTAG_RESOLUTIONUNIT, RESUNIT_INCH);
TIFFSetField(tiff, TIFFTAG_XRESOLUTION, resx);
TIFFSetField(tiff, TIFFTAG_YRESOLUTION, resy);
TIFFSetField(tiff, TIFFTAG_COMPRESSION, COMPRESSION_CCITTFAX4); //Group4 compression
TIFFSetField(tiff, TIFFTAG_IMAGEWIDTH, width);
TIFFSetField(tiff, TIFFTAG_IMAGELENGTH, height);
TIFFSetField(tiff, TIFFTAG_ROWSPERSTRIP, 1);
TIFFSetField(tiff, TIFFTAG_SAMPLESPERPIXEL, 1);
TIFFSetField(tiff, TIFFTAG_BITSPERSAMPLE, 1);
TIFFSetField(tiff, TIFFTAG_ORIENTATION, ORIENTATION_TOPLEFT);
TIFFSetField(tiff, TIFFTAG_PLANARCONFIG, PLANARCONFIG_CONTIG);
TIFFSetField(tiff, TIFFTAG_PHOTOMETRIC, PHOTOMETRIC_MINISWHITE);
tsize_t tbufsize = (width + 7) / 8; //Tiff ScanLine buffer size for 1bpp pixel row
//Now writing image to the file one row by one
int x, y;
for (y = 0; y < height; y++)
{
char *buffer = malloc(tbufsize);
memset(buffer, 0, tbufsize);
for (x = 0; x < width; x++)
{
//offset of the 1st byte of each pixel in the input image (is enough to determine is black or white in 32 bpp monochrome bitmap)
uint32 bmpoffset = ((y * width) + x) * 4;
if (bitmap[bmpoffset] == 0) //Black pixel ?
{
uint32 tiffoffset = x / 8;
*(buffer + tiffoffset) |= (0b10000000 >> (x % 8));
}
}
if (TIFFWriteScanline(tiff, buffer, y, 0) != 1)
{
return TC_ERROR_WRITING_FAILED;
}
if (buffer)
{
free(buffer);
buffer = NULL;
}
}
TIFFClose(tiff);
tiff = NULL;
return TC_SUCCESSFULL;
}
解决方案
To convert 32 bpp to 1 bpp, extract RGB and convert it into Y (luminance) and use some threshold to convert to 1 bpp.
Number of samples and bits per pixel should be 1.