在特定的应用场景下，有时候我们只是想获取图片的宽高，
但不想通过解码图片才取得这个信息。
预先知道图片的宽高信息，进而提速图片加载，预处理等相关操作以提升体验。
在*有一篇相关讨论。
Get Image size WITHOUT loading image into memory
http://*.com/questions/15800704/python-get-image-size-without-loading-image-into-memory/
不加图片到内存，而取得图像的大小。
这个技巧具有一定的实用价值，博主据此进行了相应的编码。
实现了 常用图片格式(png,jpeg,ico,bmp,gif) 不采用第三方解码库，解析得到图像宽高的函数get_image_size_without_decode_image。
bool get_image_size_without_decode_image(const char* file_path, int*width, int*height);
完整代码：
#include <stdio.h>
#include <sys/stat.h>
unsigned long byteswap_ulong(unsigned long i)
{
unsigned int j;
j = (i << );
j += (i << ) & 0x00FF0000;
j += (i >> ) & 0x0000FF00;
j += (i >> );
return j;
}
inline int Abs(int x) {
)) - (x >> );
}
unsigned short byteswap_ushort(unsigned short i)
{
unsigned short j;
j = (i << );
j += (i >> );
return j;
}
// Get Image size WITHOUT loading image into memory
// ref: http://*.com/questions/15800704/python-get-image-size-without-loading-image-into-memory/
// 博客: http://tntmonks.cnblogs.com/
// 邮箱: gaozhihan@vip.qq.com
bool get_image_size_without_decode_image(const char* file_path, int*width, int*height)
{
bool has_image_size = false;
*height = -;
*width = -;
;
FILE * fp = fopen(file_path, "rb");
if (fp == NULL)
return has_image_size;
struct stat st;
];
)
{
fclose(fp);
return has_image_size;
}
else
{
file_size = st.st_size;
}
, , fp) < )
{
fclose(fp);
return has_image_size;
}
char* png_signature = "\211PNG\r\n\032\n";
unsigned ] = { 'I', 'H', 'D', 'R' };
char* gif87_signature = "GIF87a";
char* gif89_signature = "GIF89a";
char* jpeg_signature = "\377\330";
char* bmp_signature = "BM";
) && (memcmp(sigBuf, gif87_signature, strlen(gif87_signature)) == || memcmp(sigBuf, gif89_signature, strlen(gif89_signature)) == ))
{
// image type: gif
unsigned );
*width = size_info[];
*height = size_info[];
has_image_size = true;
}
) && (memcmp(sigBuf, png_signature, strlen(png_signature)) == && memcmp(sigBuf + , ihdr_signature, strlen(ihdr_signature)) == ))
{
// image type: png
unsigned );
*width = byteswap_ulong(size_info[]);
*height = byteswap_ulong(size_info[]);
has_image_size = true;
}
) && (memcmp(sigBuf, png_signature, strlen(png_signature)) == ))
{
// image type: old png
unsigned );
*width = byteswap_ulong(size_info[]);
*height = byteswap_ulong(size_info[]);
has_image_size = true;
}
) && (memcmp(sigBuf, jpeg_signature, strlen(jpeg_signature)) == ))
{
// image type: jpeg
fseek(fp, , SEEK_SET);
;
fread(&sigBuf, , , fp);
fread(&b, , , fp);
;
;
while (b && ((unsigned char)b & 0xff) != 0xDA) {
while (((unsigned char)b & 0xff) != 0xFF)
{
fread(&b, , , fp);
}
while (((unsigned char)b & 0xff) == 0xFF) {
fread(&b, , , fp);
}
if (((unsigned char)b & 0xff) >= 0xC0 && ((unsigned char)b & 0xff) <= 0xC3)
{
fread(&sigBuf, , , fp);
fread(&sigBuf, , , fp);
unsigned short* size_info = (unsigned short*)(sigBuf);
h = byteswap_ushort(size_info[]);
w = byteswap_ushort(size_info[]);
}
else
{
unsigned ;
fread(&chunk_size, , , fp);
, SEEK_CUR) != )
break;
}
fread(&b, , , fp);
}
&& h != -)
{
*width = w;
*height = h;
}
has_image_size = true;
}
) && (memcmp(sigBuf, bmp_signature, strlen(bmp_signature)) == ))
{
// image type: bmp
unsigned ));
)
{
unsigned );
*width = size_info[];
*height = size_info[];
}
)
{
unsigned );
*width = size_info[];
*height = Abs((size_info[]));
}
has_image_size = true;
}
)
{
// image type: ico
fseek(fp, , SEEK_SET);
unsigned ;
unsigned ;
fread(&reserved, , , fp);
fread(&format, , , fp);
&& format == )
{
unsigned ;
fread(&num, , , fp);
)
{
printf("ico 包含多个图片");
}
else
{
, h = ;
fread(&w, , , fp);
fread(&h, , , fp);
*width = int((unsigned char)w & 0xff);
*height = int((unsigned char)h & 0xff);
}
}
has_image_size = true;
}
if (fp != NULL)
fclose(fp);
return has_image_size;
}
调用方法:
const char* file_path = "d:\\test.png";
int h, w;
get_image_size_without_decode_image(file_path , &w, &h);
传入图片的位置，输出对应的宽高，高和宽 为-1时，就是解析失败了。
代码比较简单，不多注释了。
若有其他相关问题或者需求可以邮件联系俺探讨。
邮箱地址是:
gaozhihan@vip.qq.com