Re: c libraries needed to deal with bitmaps - C / C++ / MFC Discussion Boards

Re: c libraries needed to deal with bitmaps

Richard MacCutchan13-Mar-20 5:22

13-Mar-20 5:22

Please use proper <pre> tags around code blocks (and proper indentation), not Quoted text. The following code sample is preceded by
<pre lang="c++">
and followed by
</pre>

C++

// Calculate the number of palette entries.
m_nPaletteEntries = 1 << m_pBIH->biBitCount;
if( m_pBIH->biBitCount > 8 )
    m_nPaletteEntries = 0;
else if( m_pBIH->biClrUsed != 0 )
    m_nPaletteEntries = m_pBIH->biClrUsed;

Much more readable.

Re: c libraries needed to deal with bitmaps

Calin Negru13-Mar-20 8:50

Calin Negru

13-Mar-20 8:50

I`m having trouble understanding how this code works. The problem is I`m not a c expert.

cf.Read( &BFH, sizeof( BITMAPFILEHEADER )

For instance how does the portion of data read from my file fits into a structure. It`s not a structure of identical items BITMAPFILEHEADER is a mix of WORDs and DWORDS

Re: c libraries needed to deal with bitmaps

Richard MacCutchan13-Mar-20 9:07

Richard MacCutchan

13-Mar-20 9:07

The sizeof operator gives the number of bytes in the object referred to. So in your case it will be the size of a BITMAPFILEHEADER, and the read operation will read the correct number of bytes into the structure. When complete each field of the structure should be correctly aligned*.

*There are conditions where this may not be true, so beware if you have a structure containing single byte elements. You may ignore that for the moment.

Re: c libraries needed to deal with bitmaps

Calin Negru13-Mar-20 22:07

Calin Negru

13-Mar-20 22:07

thanks that makes sense. In the code above do I need m_nPaletteEntries at all? Is used to calculate the offset at which individual pixels are stored?

Re: c libraries needed to deal with bitmaps

Richard MacCutchan14-Mar-20 0:04

Richard MacCutchan

14-Mar-20 0:04

You need to examine the different items in the info table as described at BITMAPINFOHEADER structure (Windows) | Microsoft Docs[^]. The values of the items related to colours will tell you whether a colour table is included or not.

Re: c libraries needed to deal with bitmaps

Calin Negru14-Mar-20 4:30

Calin Negru

14-Mar-20 4:30

I have to take a short break. Your insight (last post included) is priceless Richard, thanks

`DreamLand Page` is my projects facebook page.

Re: c libraries needed to deal with bitmaps

Richard MacCutchan14-Mar-20 4:47

Richard MacCutchan

14-Mar-20 4:47

Sorry to disappoint you, but my 'insight' is actually just a matter of reading the documentation.

Re: c libraries needed to deal with bitmaps

Calin Negru14-Mar-20 8:11

Calin Negru

14-Mar-20 8:11

the code you quoted is the bit that throws me into fog. What does the line that has the overload operator do?

Re: c libraries needed to deal with bitmaps

Richard MacCutchan14-Mar-20 8:13

Richard MacCutchan

14-Mar-20 8:13

fearless_ wrote:
What does the line that has the overload operator do?

What line?

Re: c libraries needed to deal with bitmaps

Calin Negru14-Mar-20 9:18

Calin Negru

14-Mar-20 9:18

m_nPaletteEntries = 1 << m_pBIH->biBitCount;

Re: c libraries needed to deal with bitmaps

phil.o14-Mar-20 9:37

phil.o

14-Mar-20 9:37

This is a bitwise left-shift operation which is equivalent to

C++

m_nPaletteEntries = pow(2, m_pBIH->biBitCount); // 2 to the power of bpp

What are shift operators in C++?[^]
This trick is used quite often, since a bitwise shift operation is way quicker than the pow() method, which is rather intense on processing time.

Edit: here's an interesting list of useful bitwise tricks:
Bit Twiddling Hacks[^]

"Five fruits and vegetables a day? What a joke!
Personally, after the third watermelon, I'm full."

Re: c libraries needed to deal with bitmaps

Richard MacCutchan14-Mar-20 10:27

Richard MacCutchan

14-Mar-20 10:27

It's not pow, it's multiply by 2.

Re: c libraries needed to deal with bitmaps

phil.o14-Mar-20 10:34

phil.o

14-Mar-20 10:34

For a left-shift by a single bit, yes, a multiplication by 2.
But a left-shift by n bits, in the end, is the same as a multiplication by 2 to the power of n; since original value is 1, this leads to 2 to the power of n. Or am I missing something?

"Five fruits and vegetables a day? What a joke!
Personally, after the third watermelon, I'm full."

Re: c libraries needed to deal with bitmaps

Richard MacCutchan14-Mar-20 22:58

Richard MacCutchan

14-Mar-20 22:58

I found myself thinking about this when I woke up this morning and you are, of course, quite correct.

Re: c libraries needed to deal with bitmaps

Calin Negru12-Mar-20 1:35

Calin Negru

12-Mar-20 1:35

Richard I appreciate your willingness to help

Re: c libraries needed to deal with bitmaps

Victor Nijegorodov11-Mar-20 23:45

Victor Nijegorodov

11-Mar-20 23:45

fearless_ wrote:
Would you mind helping me further? I loaded the bitmap, however when I check the values from the array into which the bitmap is loaded they show up as 0 (null).

1. How did you load the bitmap?
2. How did you "check the values from the array into which the bitmap is loaded"?
3. Did you try to open this bitmap with some graphic editor/viewer?

Re: c libraries needed to deal with bitmaps

Calin Negru12-Mar-20 1:43

Calin Negru

12-Mar-20 1:43

Victor thanks for offering to help, take a look at my reply to Richard

Re: c libraries needed to deal with bitmaps

leon de boer12-Mar-20 6:31

leon de boer

12-Mar-20 6:31

You are messing around with stuff you do not have to do on Windows and there is no advantage in doing what you are doing other than learning.

You can load a bitmap with one line of code using the API LoadBitmap()

LoadBitmapA function (winuser.h) - Win32 apps | Microsoft Docs[^]

One line of code will load it for you and it doesn't care what the bitmap format is.

HBITMAP MyBmp = LoadBitmap(0, "Yourbitmapname.bmp");

Once you have a HBITMAP (handle to a bitmap) in Windows you can basically do everything with it.
If you want the details from the bitmap once you have it loaded you just ask windows to extract them for you
So with my handle above I can extract the header with 2 lines of code the 3rd line is just to throw the details out (it assumes you are doing a console app).

BITMAP bm;
GetObject(MyBmp, sizeof(bm), &bm);
printf("Bitmap is wth: %u ht: %u bitdepth: %u\r\n", bm.bmWidth, bm.bmHeight, bm.bmBitsPixel);

So I can load the bitmap and extract the header with 3 lines in absolute safety.
If you are messing around with the actual header etc in Windows you are doing it all wrong Smile | :)

The only time you might play with a BMP file header is writing a BMP in a specific format.

That is why there is no need for libraries on Windows to handle bitmaps. Now JPEGS are a little trickier it's about 20 lines of code to get windows to load one of those most of the issue the filename has to be in UNICODE. Let me know if you need to know how to do it.

In vino veritas

modified 12-Mar-20 11:48am.

Re: c libraries needed to deal with bitmaps

Victor Nijegorodov12-Mar-20 9:39

Victor Nijegorodov

12-Mar-20 9:39

leon de boer wrote:
One line of code will load it for you and it doesn't care what the bitmap format is.
HBITMAP MyBmp = LoadBitmap(0, "Yourbitmapname.bmp");

You probaly meant the [LoadImageA function (winuser.h) - Win32 apps | Microsoft Docs](https://docs.microsoft.com/en-us/windows/win32/api/winuser/nf-winuser-loadimagea) function that can load the bitmap from the file.
LoadBitmap only loads the bitmap from resources.

Re: c libraries needed to deal with bitmaps

leon de boer12-Mar-20 14:19

leon de boer

12-Mar-20 14:19

Yep sorry needed more coffee last night, clearly Smile | :)

In vino veritas

What`s a palette in the parsing a bitmap context

Calin Negru10-Mar-20 6:00

Calin Negru

10-Mar-20 6:00

I`m learning how to work with bitmaps. Could someone explain what`s a palette when reading from a bitmap

Re: What`s a palette in the parsing a bitmap context

phil.o10-Mar-20 6:16

phil.o

10-Mar-20 6:16

A palette is a collection of indexed colours. In a bitmap using a palette, instead of refering to a colour by specifying its raw value, it is specified by its index in the palette instead.

"Five fruits and vegetables a day? What a joke!
Personally, after the third watermelon, I'm full."

Re: What`s a palette in the parsing a bitmap context

Calin Negru10-Mar-20 7:34

Calin Negru

10-Mar-20 7:34

Thanks phil.o, so this palette is used as a compression method/way to save space? Is this feature used at all? How do people usually store data to bmps with or without using a palette?

Re: What`s a palette in the parsing a bitmap context

phil.o10-Mar-20 8:05

phil.o

10-Mar-20 8:05

Exactly, this allows to save space by 1) only referencing used colours in the palette 2) using colour references instead of raw values.
This feature is used in .gif files, as well as in icons, IIRC. It is not used in .bmp files, which store colour information uncompressed. It may be used in other formats as well.

"Five fruits and vegetables a day? What a joke!
Personally, after the third watermelon, I'm full."

Re: What`s a palette in the parsing a bitmap context

harold aptroot10-Mar-20 17:31

harold aptroot

10-Mar-20 17:31

Unfortunately (I would call it that, because it's a mess) BMP files are in theory quite general, with tons of annoying features being piled onto it. Using any of those features results in a BMP file that a lot of software can't read (or reads differently than intended) because almost everyone treats BMP as only 24bit-uncompressed with no fancy extras. But anyway, in theory, a BMP file can indicate that it has 8 or fewer bits per pixel and then it must have a color table aka palette. The pixel data can even be compressed then.

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