PNG Python Reader

Complete implementation of reading png files in Python

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How does a PNG file looks like?

Below is test_png/bit_depth_32.png file. Every byte is converted to hex.
89, 50, 4e, 47, d, a, 1a, a, 0, 0, 0, d, 49, 48, 44, 52, 0, 0, 0, 3, 0, 0, 0, 3, 8, 6, 0, 0, 0, 56, 28, b5, bf, 0, 0, 0, 1, 73, 52, 47, 42, 0, ae, ce, 1c, e9, 0, 0, 0, 4, 67, 41, 4d, 41, 0, 0, b1, 8f, b, fc, 61, 5, 0, 0, 0, 9, 70, 48, 59, 73, 0, 0, e, c1, 0, 0, e, c1, 1, b8, 91, 6b, ed, 0, 0, 0, 1b, 49, 44, 41, 54, 18, 57, 63, 64, 60, f8, ff, 9f, 1, a, 98, c0, e4, 7f, a0, 18, 10, 20, c9, 30, 30, 0, 0, 6e, 34, 5, 1, 78, 6c, 90, 61, 0, 0, 0, 0, 49, 45, 4e, 44, ae, 42, 60, 82

First 8 bytes is a header of PNG which looks like:
89, 50, 4e, 47, d, a, 1a, a

When we checked if it is for sure PNG, we can read the file data.
A PNG file consists of "chunks". Every chunks contain 4 informations:

• chunk size (first 4 bytes)
• chunk type (next 4 bytes)
• chunk data (as many bits as the chunk size is)
• chunk crc (next 4 bytes)

The most important chunk types:

IHDR - start chunk

Chunk size:

• 13 bytes

Chunk data:

• image width (first 4 bytes)
• image height (next 4 bytes)
• image bit depth (next 1 byte)
• colour type: 0, 2, 3, 4 or 6 (next 1 byte)
• compression method: 0 only; deflate algorithm (next 1 byte)
• filter method: 0 only; 5 filters contain (next 1 byte)
• interlace method: 0 is no interlance, 1 is adam7 interlance (last 1 byte)

IDAT - data chunk

IEND - end chunk