binary file bit manipuation - c++

Question

I have a binary file of image data where each pixel is exactly 4 bits. Image data is laid out as follow:
There a N images where the first image is 1x1, the second image is 2x2, the third is 4x4, and so on (they are mipmaps if you care to know).
Given a pointer to the start of the data buffer, I want to skip to the biggest image.
Now I know how many bytes I want to skip, but there is this annoying 1x1 image at the start which is 4 bits. I am not aware of anyway to increment a pointer by bit.
How can I successfully retrieve the data without everything being off by 4 bits?

Answer 1

Assuming you can change your file format you can do either of the following:
Add padding to the 1x1 image
Store the images in reverse order (effectively the same as above, but not ideal for mip-maps because you don't necessarily know how many images you will have)
If you can't change your format, you have these choices:
Convert the data
Accept that the buffer is offset by half a byte and work with it accordingly
You said:
How can I successfully retrieve the data without everything being off
by 4 bits?
So that means you need to convert. When you calculate your offset in bytes, you will find that the first one contains half a byte of the previous image. So in a pinch you can shuffle them like this:
for( i = start; i < end; i++ ) {
p[i] = (p[i] << 4) | (p[i+1] >> 4);
}
That's assuming the first pixel is bits 4-7 and the second pixel is bits 0-3, and so on... If it's the other way around, just invert those two shifts.

Answer 2

// this assumes pixels points to bytes(unsigned chars)
index = ?;// your index to the pixel
byte_t b = pixels[index / 2];
if (index % 2) pixel = b >> 4;
else pixel = b & 15;
// Or you can use
byte_t b = pixels[index >> 1];
if (index & 1) pixel = b >> 4;
else pixel = b & 15;
Either way just compute the logical index into the file. Dividing by two takes you to the start of the byte where the pixel is. And then just read the correct half of the byte.
So make a function
byte_t GetMyPixel(unsigned char* pixels, unsigned index) {
byte_t b = pixels[index >> 1];
byte_t pixel;
if (index & 1) pixel = b >> 4;
else pixel = b & 15;
return pixel;
}
To read first image.
Image1x1 = GetMyPixel(pixels,0);
Image2x2_1 = GetMyPixel(pixels,1);// Top left pixel of second image
Image2x2_2 = GetMyPixel(pixels,2);// Top Right pixel of second image
Image2x2_3 = GetMyPixel(pixels,3);// Bottom left pixel of second image
... etc
So that is one way to go about it. You might need to take into account the endian-ness you are using so if it seems wrong then switch the logic for the pixel read thusly...
byte_t GetMyPixel(unsigned char* pixels, unsigned index) {
byte_t b = pixels[index >> 1];
byte_t pixel;
#if OTHER_ENDIAN
if (index & 1) pixel = b >> 4;
else pixel = b & 15;
#else
if (index & 1) pixel = b & 15;
else pixel = b >> 4;
#endif
return pixel;
}