We are trying to build a PNG Encoder. We have problems understanding how is the IDAT chunk built. We understand that the first 4 bytes are the length,the second 4 bytes are the IDAT signature, then raw data and of course 4 bytes of CRC at the end. Our problem is that the first 7 bytes in the data do not represent pixel indexing, but something else, and we can't figure out what. Any ideas what those bytes might represent?
Any help would be appreciated