Question

How to Check if an image is valid

For example, when I cached an image which was downloaded only half and failed, and this image is invalid, then I want to know it’s an invalid image and download again.
(When I use the broken image, Xcode console logs an error:
ImageIO: PNG IDAT: CRC error )

So I want find a mechanism to check image’s validation
for different kinds of JPEG,PNG,etc

anybody has some clues?

Answer 1

I would start by checking for valid header.
Secondly the footer. Usually the last 8 bytes uint values in order are
‘73,69,78,68,174,66,96,130’.
This converted into a Int64 equals 5279712195050102914

This should do for a png 🙂

if (memcmp(img_bytes, “\211PNG”, 4) != 0||OSReadBigInt64(img_bytes,(length – 8))!=5279712195050102914)

{

  //Bad Data! Free your data and return or something

}