How does Bitmap.Save(Stream, ImageFormat) format the data?

Question

I have a non transparent, colour bitmap with length 2480 and width 3507.

Using Bitmap.GetPixel(int x, int y) I am able to get the colour information of each pixel in the bitmap.

If I squirt the bitmap into a byte[]:

MemoryStream ms = new MemoryStream();
bmp.Save(ms, ImageFormat.Bmp);
ms.Position = 0;
byte[] bytes = ms.ToArray();

then I'd expect to have the same information, i.e. I can go to bytes[1000] and read the colour information for that pixel.

It turns out that my array of bytes is larger than I anticipated. I thought I'd get an array with 2480 x 3507 = 8697360 elements. Instead I get an array with 8698438 elements - some sort of header I presume.

In what format the bytes in my array stored? Is there a header 1078 bytes long followed by Alpha, Red, Green, Blue values for every byte element, or something else?

All I need is the colour information for each pixel. I'm not concerned with the header (or indeed the transparency) unless I need it to get the colour info.

Answer 1

You're calling GetBuffer which returns the underlying byte array - that's bigger than the actual length of the stream.

Either use

byte[] bytes = ms.ToArray();

or use GetBuffer but in conjunction with ms.Length.

Having said that, you're saving it as a BMP - so there'll be header information as well; it's not like the first byte will represent the first pixel. Unfortunately there's no "raw" image format as far as I can see... that's what it sounds like you really want.

You could use Bitmap.LockBits and then copy the data from there, if you want...

Answer 2

If I understand correctly the documentation about Bitmap.Save, it saves the image in the specified format, which means that you'll have in your bytes array, the bitmap header. I think you should read some documentation about the bitmap format to know how to get the needed information in your array