Alternative to BitConverter.ToInt32

Question

I'm using BitConverter.ToInt32 to pack 3 byte values into an int, like so:

byte R = 0;
byte G = 0;
byte B = 0;
int i = BitConverter.ToInt32(new byte[] { R, G, B, 0 }, 0);

Is there a faster way to do this that doesn't involve the creation of a new int each time? Getting the bytes out of an int is easy:

int i = 34234;
byte B = (byte)(i >> 0);
byte G = (byte)(i >> 8);
byte R = (byte)(i >> 16);

Is there a simple way to reverse this process and use bit-shifting to write the RGB bytes back over an existing int?

Why are you worried about the creation of a new `int`? Creating an `int` is free. The part to be concerned about is creation of a new `byte[]` every time. — Gabe, Mar 28 '10 at 16:59
@gabe: this is just sample code. In the real code, I was assigning to a pre-instantiated class-level byte array. Creating an int is virtually free, but not if you're doing this for hundreds of 320 x 240 arrays. The bit-shifting is significantly faster than BitConverter.ToInt32. — MusiGenesis, Mar 28 '10 at 17:03
You're convoluting RGB and BGR here (the two code snippets do not round-trip). — Ben Voigt, Mar 28 '10 at 17:23
Creating an int isn't "virtually" free -- it's completely free. BitConverter isn't slow. Putting the bytes into the array and making the functino call is what's slow. — Gabe, Mar 28 '10 at 17:52
I finally did some benchmarking on this: http://stackoverflow.com/questions/4326125/faster-way-converter-byte-array-to-int/4331638#4331638 It seems that the function call overhead of `BitConverter` is fairly significant. — Gabe, Dec 02 '10 at 06:13

score 9 · Accepted Answer · answered Mar 28 '10 at 16:55

9

int i = (B << 0) | (G << 8) | (R << 16);

answered Mar 28 '10 at 16:55

dtb

Shifting by 0 is not really necessary. – IVlad Mar 28 '10 at 17:00
Love the Lemmings avatar, by the way. I think I mentioned that before. – MusiGenesis Mar 28 '10 at 17:04
4

@IVlad: But it makes the code much better to read, at no cost. – H H Mar 28 '10 at 17:15
2

This undoes the bit-shifting shown, which is in BGR(A) format. But it isn't equivalent to the BitConverter snippet, which uses RGB(A) format. – Ben Voigt Mar 28 '10 at 17:25

score 4 · Answer 2 · answered Mar 28 '10 at 16:56

4

You ought to consider the Color structure. It has R, G and B properties and FromArgb() and ToArgb() methods.

answered Mar 28 '10 at 16:56

Hans Passant

3

You'd think `Color` would be ideal for this, but it's piggishly slow. – MusiGenesis Mar 28 '10 at 17:00
1

Hard to see how it could be, it uses simple shifts, and and or, just like regular code. – Hans Passant Mar 28 '10 at 17:11
2

I can confirm Musi's findings. Color.FromRgb is even slower than the BitConverter. – H H Mar 28 '10 at 17:40

2 Answers2