不用第三方解码库取得图片宽高 附完整C++算法实现代码

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stackoverflow有一篇相关讨论。

Get Image size WITHOUT loading image into memory
http://stackoverflow.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。

项目地址:https://github.com/cpuimage/image_size

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,int *filesize);

完整代码：

#include <stdio.h>
#include <stdlib.h>
#include <sys/stat.h>
#include <stdbool.h>
#include <stdint.h>

#if defined(_MSC_VER) && (_MSC_VER >= 1310) /*Visual Studio: A few warning types are not desired here.*/
#pragma warning( disable : 4996 ) /*VS does not like fopen, but fopen_s is not standard C so unusable here*/
#endif /*_MSC_VER */

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) {
    return  (x ^ (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://stackoverflow.com/questions/15800704/python-get-image-size-without-loading-image-into-memory/

bool get_image_size_without_decode_image(const char* file_path, int*width, int*height, int * file_size)
{
    bool has_image_size = false;
    *height = -;
    *width = -;
    *file_size = -;
    FILE * fp = fopen(file_path, "rb");
    if (fp == NULL)
        return has_image_size;
    struct stat st;
    char sigBuf[];
    if (fstat(fileno(fp), &st) < )
    {
        fclose(fp);
        return has_image_size;
    }
    else
    {
        *file_size = st.st_size;
    }
    if (fread(&sigBuf, , , fp) < )
    {
        fclose(fp);
        return has_image_size;
    }
    char* png_signature = "\211PNG\r\n\032\n";
    char* ihdr_signature = "IHDR";
    char* gif87_signature = "GIF87a";
    char* gif89_signature = "GIF89a";
    char* jpeg_signature = "\377\330";
    char* bmp_signature = "BM";
    if ((*file_size >= ) && (memcmp(sigBuf, gif87_signature, strlen(gif87_signature)) ==  || memcmp(sigBuf, gif89_signature, strlen(gif89_signature)) == ))
    {
        // image type: gif
        unsigned short* size_info = (unsigned short*)(sigBuf + );
        *width = size_info[];
        *height = size_info[];
        has_image_size = true;
    }
    else if ((*file_size >= ) && (memcmp(sigBuf, png_signature, strlen(png_signature)) ==  && memcmp(sigBuf + , ihdr_signature, strlen(ihdr_signature)) == ))
    {
        // image type:   png
        unsigned long* size_info = (unsigned long*)(sigBuf + );
        *width = byteswap_ulong(size_info[]);
        *height = byteswap_ulong(size_info[]);
        has_image_size = true;
    }
    else if ((*file_size >= ) && (memcmp(sigBuf, png_signature, strlen(png_signature)) == ))
    {
        // image type: old png
        unsigned long* size_info = (unsigned long*)(sigBuf + );
        *width = byteswap_ulong(size_info[]);
        *height = byteswap_ulong(size_info[]);
        has_image_size = true;
    }
    else if ((*file_size >= ) && (memcmp(sigBuf, jpeg_signature, strlen(jpeg_signature)) == ))
    {
        // image type: jpeg
        fseek(fp, , SEEK_SET);
        char b = ;
        fread(&sigBuf, , , fp);
        fread(&b, , , fp);
        int w = -;
        int h = -;
        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 short chunk_size = ;
                fread(&chunk_size, , , fp);
                if (fseek(fp, byteswap_ushort(chunk_size) - , SEEK_CUR) != )
                    break;
            }
            fread(&b, , , fp);
        }
        if (w != - && h != -)
        {
            *width = w;
            *height = h;
        }
        has_image_size = true;
    }
    else if ((*file_size >= ) && (memcmp(sigBuf, bmp_signature, strlen(bmp_signature)) == ))
    {
        // image type: bmp
        unsigned int header_size = (*(sigBuf + ));
        if (header_size == )
        {
            unsigned short* size_info = (unsigned short*)(sigBuf + );
            *width = size_info[];
            *height = size_info[];
        }
        else if (header_size >= )
        {
            unsigned int* size_info = (unsigned int*)(sigBuf + );
            *width = size_info[];
            *height = Abs((size_info[]));
        }
        has_image_size = true;
    }
    else if (*file_size >= )
    {
        // image type: ico
        fseek(fp, , SEEK_SET);
        unsigned short format = -;
        unsigned short reserved = -;
        fread(&reserved, , , fp);
        fread(&format, , , fp);
        if (reserved ==  && format == )
        {
            unsigned short num = -;
            fread(&num, , , fp);
            if (num > )
            {
                printf("this is a muti-ico file.");
            }
            else
            {
                char w = , 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;
}

int main(int argc, char ** argv) {

    const char * pic_file = "lookmanodeps.png";
    int w = ;
    int h = ;
    int filesize = ;
    get_image_size_without_decode_image(pic_file, &w, &h, &filesize);
    printf("file:%s\nfilesize:%d\nwidth:%d\nheight%d\n", pic_file, filesize, w, h);
    getchar();
}

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。

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gaozhihan@vip.qq.com