24位bitmap

bmp位图格式有好几种，结构上稍有不同。下面只针对标准的24位位图。
一个标准的24位位图，由54字节的头，和位图数据组成。
其中头分为3部分：2字节的效验字符，12字节文件头，40字节文件信息。

• 效验字符一定为大写字母BM的ascii码。
• 文件头分为3部分：
  • 前4字节是一个无符号的32位整数，表示文件大小；
  • 中间4字节位保留字段，目前用途不明，一般为0；
  • 后4字节位32是一个无符号的32位整数，表示位图数据的偏移量，一般为54。
• 文件信息共11个字段，其中只有两个字段为2字节， 其余均为4字节。详细见结构体定义。其一般取值见函数。

struct bmp_magic
{
	char magic[2];
};

struct bmp_head
{
	unsigned file_size;
	unsigned short creater1;
	unsigned short creater2;
	unsigned offset;
};

struct bmp_info
{
	unsigned	header_size;
	unsigned	width;
	unsigned	height;
	unsigned short	nplanes;
	unsigned short	bitspp;
	unsigned	compress_type;
	unsigned	image_size;
	unsigned	hres;
	unsigned	vres;
	unsigned	ncolors;
	unsigned	nimpcolors;
};


void write_bmp_head(FILE *fp, unsigned short w, unsigned short h)
{
	unsigned char buffer[54];
	unsigned skip = (unsigned) w * 3;
	while (skip % 4) ++skip;
	
	buffer[0] = 'B';
	buffer[1] = 'M';
	
	bmp_head *head = (bmp_head *) (buffer + 2);
	head->file_size = skip * h + 54;
	head->creater1 = 0;
	head->creater2 = 0;
	head->offset = 54;

	bmp_info *info = (bmp_info *) (buffer + 14);
	info->header_size = 40;
	info->width = w;
	info->height = h;
	info->nplanes = 1;
	info->bitspp = 24;
	info->compress_type = 0;
	info->image_size = skip * h;
	info->hres = 0;
	info->vres = 0;
	info->ncolors = 0;
	info->nimpcolors = 0;

	fwrite(buffer, 54, 1, fp);
}

void write_bmp_data(FILE *fp, unsigned short w, unsigned short h, unsigned *bitmap)
{
	unsigned pad = (4 - w * 3 % 4) % 4;
	unsigned padding = 0;
	unsigned *p;
	
	bitmap += w * h;
	while (h > 0)
	{
		p = bitmap - w;
		while (p < bitmap)
		{
			fwrite(p, 3, 1, fp);
			++p;
		}
		if (pad) fwrite(&padding, pad, 1, fp);
		bitmap -= w;
		--h;
	}
}

void read_bmp_data(FILE *fp, unsigned short w, unsigned short h, unsigned *bitmap)
{
	unsigned pad = (4 - w * 3 % 4) % 4;
	unsigned padding = 0;
	unsigned *p;
	
	bitmap += w * h;
	while (h > 0)
	{
		p = bitmap - w;
		while (p < bitmap)
		{
			fread(p, 3, 1, fp);
			++p;
		}
		if (pad) fread(&padding, pad, 1, fp);
		bitmap -= w;
		--h;
	}
}