I'm going over the W3 PNG specification (creating a PNG library from scratch) and I've finally found how to create a green 1x1 image.

Now I'm trying to create a bigger image of mixed Red, Green and Blue pixels. Let's say a 4x4 image. Sadly I'm getting all the pixels mixed and some of them are Black or Pink.


  • Signature: OK
  • IHDR: OK
    • Width: 4
    • Heigh: 4
    • Bit depth: 8
    • Color: 2
    • Filter: 0
    • Compression: 0
    • Interlace: 0
  • IDATA: 4 chunks*
  • IEND: OK

*IDATA chunks:

  • 4 bytes length: Number of bytes of the DATA after Zlib deflate
  • 4*1 byte type: IDAT
  • X bytes data: 4*4bytes unsigned integers, Zlib deflated, one after the other, Network byte order
    • 255 for blue**
    • 65280 for green**
    • 16711680 for red**
  • 4 bytes crc: OK

**Bitwise results of:

alpha<<24 | red<<16 | green<<8 | blue

With alpha, blue, green and red taking values from 0 to 255

What's wrong with this?


1 Answer 1


I just answered this in Stack Overflow. Here's the answer again:

The PNG pixels are in RGBA order, not ARGB, so you'd need to write

alpha | red<<24 | green << 16 | blue <<8

But you are writing color type 2, so your pixels should be 3 bytes each instead of four; you can't really encode them in 4-byte integers as you've done. So either change the color type to 6 or switch to writing each sample as an individual byte.

Also you need a filter byte at the beginning of each row. Zero will work for you. So in your example 4x4 RGB image you will need to write 13 bytes per row

0 R G B R G B R G B R G B
0 R G B R G B R G B R G B
0 R G B R G B R G B R G B
0 R G B R G B R G B R G B

and zlib-compress that. It can all go into one IDAT chunk. If you need to write smaller IDAT chunks, you have to zlib-compress the image first, then split the zlib output into pieces that you put in consecutive IDAT chunks.


Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Not the answer you're looking for? Browse other questions tagged or ask your own question.