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#include #include #include #include #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 << 24); j += (i << 8) & 0x00FF0000; j += (i >> 8) & 0x0000FF00; j += (i >> 24); return j; } inline int Abs(int x) { return (x ^ (x >> 31)) - (x >> 31); } unsigned short byteswap_ushort(unsigned short i) { unsigned short j; j = (i << 8); j += (i >> 8); 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 = -1; *width = -1; *file_size = -1; FILE * fp = fopen(file_path, "rb"); if (fp == NULL) return has_image_size; struct stat st; char sigBuf[26]; if (fstat(fileno(fp), &st) < 0) { fclose(fp); return has_image_size; } else { *file_size = st.st_size; } if (fread(&sigBuf, 26, 1, fp) < 1) { 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 >= 10) && (memcmp(sigBuf, gif87_signature, strlen(gif87_signature)) == 0 || memcmp(sigBuf, gif89_signature, strlen(gif89_signature)) == 0)) { // image type: gif unsigned short* size_info = (unsigned short*)(sigBuf + 6); *width = size_info[0]; *height = size_info[1]; has_image_size = true; } else if ((*file_size >= 24) && (memcmp(sigBuf, png_signature, strlen(png_signature)) == 0 && memcmp(sigBuf + 12, ihdr_signature, strlen(ihdr_signature)) == 0)) { // image type: png unsigned long* size_info = (unsigned long*)(sigBuf + 16); *width = byteswap_ulong(size_info[0]); *height = byteswap_ulong(size_info[1]); has_image_size = true; } else if ((*file_size >= 16) && (memcmp(sigBuf, png_signature, strlen(png_signature)) == 0)) { // image type: old png unsigned long* size_info = (unsigned long*)(sigBuf + 8); *width = byteswap_ulong(size_info[0]); *height = byteswap_ulong(size_info[1]); has_image_size = true; } else if ((*file_size >= 2) && (memcmp(sigBuf, jpeg_signature, strlen(jpeg_signature)) == 0)) { // image type: jpeg fseek(fp, 0, SEEK_SET); char b = 0; fread(&sigBuf, 2, 1, fp); fread(&b, 1, 1, fp); int w = -1; int h = -1; while (b && ((unsigned char)b & 0xff) != 0xDA) { while (((unsigned char)b & 0xff) != 0xFF) { fread(&b, 1, 1, fp); } while (((unsigned char)b & 0xff) == 0xFF) { fread(&b, 1, 1, fp); } if (((unsigned char)b & 0xff) >= 0xC0 && ((unsigned char)b & 0xff) <= 0xC3) { fread(&sigBuf, 3, 1, fp); fread(&sigBuf, 4, 1, fp); unsigned short* size_info = (unsigned short*)(sigBuf); h = byteswap_ushort(size_info[0]); w = byteswap_ushort(size_info[1]); } else { unsigned short chunk_size = 0; fread(&chunk_size, 2, 1, fp); if (fseek(fp, byteswap_ushort(chunk_size) - 2, SEEK_CUR) != 0) break; } fread(&b, 1, 1, fp); } if (w != -1 && h != -1) { *width = w; *height = h; } has_image_size = true; } else if ((*file_size >= 26) && (memcmp(sigBuf, bmp_signature, strlen(bmp_signature)) == 0)) { // image type: bmp unsigned int header_size = (*(sigBuf + 14)); if (header_size == 12) { unsigned short* size_info = (unsigned short*)(sigBuf + 18); *width = size_info[0]; *height = size_info[1]; } else if (header_size >= 40) { unsigned int* size_info = (unsigned int*)(sigBuf + 18); *width = size_info[0]; *height = Abs((size_info[1])); } has_image_size = true; } else if (*file_size >= 2) { // image type: ico fseek(fp, 0, SEEK_SET); unsigned short format = -1; unsigned short reserved = -1; fread(&reserved, 2, 1, fp); fread(&format, 2, 1, fp); if (reserved == 0 && format == 1) { unsigned short num = -1; fread(&num, 2, 1, fp); if (num > 1) { printf("this is a muti-ico file."); } else { char w = 0, h = 0; fread(&w, 1, 1, fp); fread(&h, 1, 1, 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 = 0; int h = 0; int filesize = 0; 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。
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