View Full Version : Finding bits of Bitmap

05-15-2003, 01:17 AM

I am trying to find the bits of a bit map so I can mess with the Alpha values.

I have seen this done using oleLoadPicturePath - Nehe style but I do not want to use this as it seems to be overkill when LoadImage will do...is there a simplier way to do it i.e. using GetDIBits?



05-15-2003, 02:38 AM
GetDIBits(...) will work fine.

For storing alpha values a 32 bit pixel format is needed. But GetDIBits(...) failed when I tested by setting 32 to the pixelbits member of the BITMAPINFOHEADER structure . Precise results were obtained
by setting 24 to the pixelbits member and then expanding to output byte array to
32 bit pixel format.

If you need any more help i can send the working code.

05-15-2003, 03:53 AM
That's probably why it isn't currently working then!

So I have to read in my BMP file, then call GetDIBits using a BITMAPINFO structure with a depth of 24bits and then add in the extra Alpha bits?


05-15-2003, 07:46 AM
erm...still trying...could you send the code?

How do I set up the INFOHEADER? Looking @ MSDN there are a few versions http://www.opengl.org/discussion_boards/ubb/smile.gif

Mine keeps giving me errors...I think it maybe to do with my buffer area - i.e. where the bits are stored after reading them.

Dunno though http://www.opengl.org/discussion_boards/ubb/smile.gif

05-15-2003, 08:11 AM
Ok got it now...went onto MSDN online and found some useful stuff on there...

can anyone explain what the hell is going on here though?

biSizeImage = ((((biWidth * biBitCount) + 31) &
~31) >> 3) * biHeight:


05-15-2003, 09:22 AM
biSizeImage =
((((biWidth * biBitCount) + 31) &
~31) >> 3) * biHeight:

The above mentioned statement is for calculating the imagesize in terms of bytes.

In terms of bytes the row length of bitmap must always be a multiple of four and to set this often extra padding bytes are added at the end of the row.

The row length with padding bytes is calculated as follows
((((biWidth * biBitCount) + 31) & ~31) >> 3)

05-15-2003, 02:14 PM
Ok, so I have my bytes...

As I understand it the whole bitmaps bits are in a DWORD array. Each "RGB" chunk can be retrieved by "casting" each element into a BYTE, thus giving 4 BYTES to each DWORD element in the array...I think...with it in the order BGRA instead of the standard RGBA.

Now when for a test I copy these values into another array and pass this to glTexImage2D and this works. So I though it would be cool to re-arrange the bits so they are RGBA by simply copying B -> temp and then R -> B and finally temp -> R, i.e. swap R and B via a temp variable...this don't work.

Also it seems that what I thought was the alpha BYTE...pun not intended...is not the case!

Could anyone explain how the BYTES are arranged? Or am I way off the mark here?


05-16-2003, 12:09 AM
The bitmap bits out of GetDIBits(...) is stored only in the BYTE array and not in a DWORD array. The values from the BYTE array can be easiy copied into the second BYTE array without any typecasing. Only the red and blue values have to be swapped