View source for Load BMP data (for heightmaps etc)

=== Overview ===

BMP is a simple image format developed by Microsoft. The colordepth wary between 2 (1-bit), 16 (4-bit), 256 (8-bit), 16-bit and 24-bit. The image is usually not compressed, but it might be compressed with the RLE algorithm. This tutorial will cover how to read pixels from a BMP image into an 2-dimensional array.

=== File format ===

==== File header ====

{| 
! Offset !! Bytes !! Description !! C/C++ datatype
|-
| 0x00 - 0x01 || 2 || This bytes must contain the ASCII character 'B' and 'M' || Char
|-
| 0x02 - 0x05 || 4 || Size of the file in bytes || Integer
|-
| 0x06 - 0x07 || 4 || Reserved, must be zero || 
|-
| 0x08 - 0x09 || 4 || Reserved, must be zero ||  
|-
| 0x0A - 0x0D || 4 || Offset to pixeldata || Integer
|}

==== Image header ====

{| 
! Offset !! Bytes !! Description !! C/C++ datatype
|-
| 0x0E - 0x11 || 4 || Size of the image header, must be 40 (0x28) || Integer
|-
| 0x12 - 0x15 || 4 || Image width in pixels || Integer
|-
| 0x16 - 0x19 || 4 || Image height in pixels || Integer
|-
| 0x1A - 0x1B || 4 || Nr of planes, must be one || Integer
|-
| 0x1C - 0x1D || 4 || Bits per pixel, must be 1,2,4,8,16,24 or 32 || Integer
|-
| 0x1E - 0x21 || 4 || Compression
0 = No compression
1 = RLE compression for 8bit bitmaps
2 = RLE compression for 4bit bitmaps 	Integer
Following fields may be ignored most of the time. They is used to describe and store the palette. || 
|-
| 0x22 - 0x25 || 4 || Size of image data, may be zero if no compression is used || Integer
|-
| 0x26 - 0x29 || 4 || Horizontal resolution of the image || Integer
|-
| 0x2A - 0x2D || 4 || Vertical resolution of the image || Integer
|-
| 0x2E - 0x31 || 4 || Number of color indexes actually used, if set to zero all colors is used || Integer
|-
| 0x32 - 0x35 || 4 || Nr of colors indexes required to render the image, if set to zero all color is used || Integer
|-
| 0x36 - || 4 || Optional: This part may contain the palette used by the image.
The format is RGBx (x is 1 byte reserved, must be set to zero) || 
|}

=== Reading the header ===

The first step to read our image is to read its header to find out its size,bits-per-pixel and encoding. We will begin by making a simple struct to hold our data and a couple of enumerations for eventual errors. Lets start with struct.

<codebox>
typedef struct Bitmap {
	int offset;
	int width;
	int height;
	int bpp;
	int encoding;
	int size;
	bool loaded;
	unsigned char* pData;
};
</codebox>

For now this is all we need to store, but we will add more later. I don't think any of these need any further explanation.

<codebox>
enum BITMAP_RESULTS {
	BMP_OK,
	BMP_NO_FILE,
	BMP_INVALID_HEADER,
	BMP_OUT_OF_MEMORY,
	BMP_UNSUPPORTED,
	BMP_LOADED,
};
</codebox>

The first enum simply indicates everythings good soo far. BMP_NO_FILE means the file could not be opened, most likely the file doesn't exist but it might be permission error. BMP_INVALID_HEADER tells us the file isn't a valid BMP file, either it doesn't start with BM or the length is wrong. BMP_OUT_OF_MEMORY is self explanary. BMP_UNSUPPORTED is going to be used when the bpp has weired modes and/or encodings. The last one, BMP_LOADED, is used when there is already a bitmap loaded.

In order to read the header we first need to load the file into memory:

<codebox>
#include <fstream>

using namespace std;

int loadBMP(Bitmap* img,char* filename){
	ifstream::pos_type size
	ifstream file(filename, ios::in|ios::binary|ios::ate);
	if(file.is_open()){
		size = file.tellg();
		memblock = new char [size];
		file.seekg (0, ios::beg);
		file.read (memblock, size);
		file.close();
	}
}
</codebox>

=== Loading 8-bit monocrome image ===

TBD

=== Loading 24-bit truecolor image ===

TBD

=== Summary ===

TBD

[[Category:OpenGL]]