Question

How can I write only every third item in a char buffer to file quickly in C++?

I get a three-channel image from my camera, but each channel contains the same info (the image is grayscale). I’d like to write only one channel to disk to save space and make the writes faster, since this is part of a real-time, data collection system.

C++’s ofstream::write command seems to only write contiguous blocks of binary data, so my current code writes all three channels and runs too slowly:

char * data = getDataFromCamera();
int dataSize = imageWidth * imageHeight * imageChannels;
std::ofstream output;
output.open( fileName, std::ios::out | std::ios::binary );
output.write( data, dataSize );

I’d love to be able to replace the last line with a call like:

int skipSize = imageChannels;
output.write( data, dataSize, skipSize );

where skipSize would cause write to put only every third into the output file. However, I haven’t been able to find any function that does this.

I’d love to hear any ideas for getting a single channel written to disk quickly.
Thanks.

Answer 1

Let’s say your buffer is 24-bit RGB, and you’re using a 32-bit processor (so that operations on 32-bit entities are the most efficient).

For the most speed, let’s work with a 12-byte chunk at a time. In twelve bytes, we’ll have 4 pixels, like so:

AAABBBCCCDDD

Which is 3 32-bit values:

AAAB
BBCC
CDDD

We want to turn this into ABCD (a single 32-bit value).

We can create ABCD by applying a mask to each input and ORing.

ABCD = A000 | 0BC0 | 000D

In C++, with a little-endian processor, I think it would be:

unsigned int turn12grayBytesInto4ColorBytes( unsigned int buf[3] )
{
   return (buf[0]&0x000000FF) // mask seems reversed because of little-endianness
        | (buf[1]&0x00FFFF00)
        | (buf[2]&0xFF000000);
}

It’s probably fastest to do this another conversion to another buffer and THEN dump to disk, instead of going directly to disk.