Thread: Loading a bitmap

Originally Posted by brewbuck

This is why you should not use sizeof() on a struct to load packed binary data from a file. It ignores alignment and endian issues.

You can't just imagine that the struct overlays perfectly on a section of file data. Things don't work that way.

And removing sizeof() will solve exactly which part of the problem? I'm sure you mean the right thing, but sizeof() itself is not a culprit in this case - the struct itself holds 16 bit and 32 bit data intermixed in ways that cause the compiler to add padding - in the middle of the struct. So alignment is very much the issue here.

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Mats

Originally Posted by matsp

And removing sizeof() will solve exactly which part of the problem? I'm sure you mean the right thing, but sizeof() itself is not a culprit in this case - the struct itself holds 16 bit and 32 bit data intermixed in ways that cause the compiler to add padding - in the middle of the struct. So alignment is very much the issue here.

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Mats

I didn't say that sizeof() was the problem. I (implicitly) was saying that the whole concept of taking a struct, laying it out like some portion of a file format, then using sizeof() to compute its size and read it as a single chunk, is wrong.

Further, if you follow the rule "never use sizeof() while reading a file format" then you eliminate a whole class of errors, like the aforementioned, as well as assuming a particular endianness and integer width. Without sizeof(), you are forced to read a single byte at a time, which forces you to deal with endianness and width from the very outset instead of ignoring them.

These are the structs provided by Microsoft.

Code:

typedef struct tagBITMAPFILEHEADER { 
  WORD    bfType; 
  DWORD   bfSize; 
  WORD    bfReserved1; 
  WORD    bfReserved2; 
  DWORD   bfOffBits; 
} BITMAPFILEHEADER, *PBITMAPFILEHEADER;

typedef struct tagBITMAPINFOHEADER{
  DWORD  biSize; 
  LONG   biWidth; 
  LONG   biHeight; 
  WORD   biPlanes; 
  WORD   biBitCount; 
  DWORD  biCompression; 
  DWORD  biSizeImage; 
  LONG   biXPelsPerMeter; 
  LONG   biYPelsPerMeter; 
  DWORD  biClrUsed; 
  DWORD  biClrImportant; 
} BITMAPINFOHEADER, *PBITMAPINFOHEADER;

Which equal:

Code:

typedef struct tagBITMAPFILEHEADER { 
  unsigned short bfType; 
  unsigned long  bfSize; 
  unsigned short bfReserved1; 
  unsigned short bfReserved2; 
  unsigned long  bfOffBits; 
} BITMAPFILEHEADER, *PBITMAPFILEHEADER;

typedef struct tagBITMAPINFOHEADER{
  unsigned long  biSize; 
  long           biWidth; 
  long           biHeight; 
  unsigned short biPlanes; 
  unsigned short biBitCount; 
  unsigned long  biCompression; 
  unsigned long  biSizeImage; 
  long           biXPelsPerMeter; 
  long           biYPelsPerMeter; 
  unsigned long  biClrUsed; 
  unsigned long  biClrImportant; 
} BITMAPINFOHEADER, *PBITMAPINFOHEADER;

I remember I have used these once to read bitmap files. Microsoft itself uses this way too...

And GameDev has this as a tutorial too: http://www.gamedev.net/reference/art...rticle1966.asp

And by the way, MS also does:

Code:

#include <pshpack2.h>
typedef struct tagBITMAPFILEHEADER {
....

pshpack2.h in turn does:
#pragma pack(2)

[It's a bit more complex than that - it tests for differnet compiler versions and a bunch of other #if type things before it does this, but that's what it amounts to].

You would have seen that if you read the line just above the header you copied and pasted from WinGDI.h - or did you just copy'n'paste from some other place that doesn't show the context of the structure definition?

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Mats