First, you get a NullReferenceException because bmpBinary is NULL.
Second, to get one Color chanel you can use int red = Color.red(pixel);
Third, to set a pixel white use bmpBinary.setPixel(x, y, 0xFFFFFFFF);
I modified your code a bit:
public Bitmap toBinary(Bitmap bmpOriginal) {
int width, height, threshold;
height = bmpOriginal.getHeight();
width = bmpOriginal.getWidth();
threshold = 127;
Bitmap bmpBinary = Bitmap.createBitmap(bmpOriginal);
for(int x = 0; x < width; ++x) {
for(int y = 0; y < height; ++y) {
// get one pixel color
int pixel = bmpOriginal.getPixel(x, y);
int red = Color.red(pixel);
//get binary value
if(red < threshold){
bmpBinary.setPixel(x, y, 0xFF000000);
} else{
bmpBinary.setPixel(x, y, 0xFFFFFFFF);
}
}
}
return bmpBinary;
}
An even better way is not to use just the value of one color chanel but a weighted average of red green and blue for example:
int gray = (int)(red * 0.3 + green * 0.59 + blue * 0.11);