这种方法效率低下

NSData *data = UIImagePNGRepresentation(image);

NSMutableString *picStr = [NSMutableString stringWithString:@"["];

Byte *dataByte = (Byte *)[data bytes];

int length = [data length];

//字节流转nsstring；

for(int i = 0;i < length;i++){

[picStr appendFormat:@"%d,",(int8_t)dataByte[i]];

}