javascript - Determine if a base64 string or a buffer contains JPEG or PNG without metadata? Possible?

Question

Is there any way to do this using node, whether natively or with a plugin?

What I'm trying to accomplish is to choose loseless or lossy image compression depending on the input type. Loseless on a large JPEG is a storage catastrophe.

Answer 1

The first eight bytes of a PNG file always contain the following values - see PNG Specification:

(decimal)              137  80  78  71  13  10  26  10
(hexadecimal)           89  50  4e  47  0d  0a  1a  0a
(ASCII C notation)    211   P   N   G  
  
 32 

So, if I take 8 bytes from the start of any PNG file and base64 encode it as follows, I get:

head -c8 test.png | base64
iVBORw0KGgo=

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The first 2 bytes of every JPEG file contain ff d8 in hex - see Wikipedia entry for JPEG. So if I take any JPEG file and base64 encode the first two bytes as follows, I get:

head -c2 test.jpg | base64
/9g=

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So my suggestion would be to look at the first few (10 for PNG and 2 for JPEG, always excluding the =) characters of your base64-encoded file and see if they match what I am suggesting and then use that as the determinant - be sure to output error messages if your string matches neither in case the test is not sufficiently thorough for some reason!

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Why 10 characters for PNG? Because the guaranteed signature is 8 bytes, i.e. 64 bits and base64 splits into 6 bits at a time to generate a character, so the first 10 characters are the first 60 bits. The 11th character will vary depending on what follows the signature.

Same logic for JPEG... 2 bytes is 16 bits, which means 2 characters each corresponding to 6 bits are guaranteed. The 3rd character will vary depending on what follows the 2-byte SOI marker.