I looked at the latest github release for save.c and found this for the header.
outputData[0] = 'O';
outputData[1] = 'P';
outputData[2] = 'S';
outputData[3] = '1';
outputData[4] = SAVE_VERSION;
outputData[5] = CELL;
outputData[6] = blockW;
outputData[7] = blockH;
outputData[8] = finalDataLen;
outputData[9] = finalDataLen >> 8;
outputData[10] = finalDataLen >> 16;
outputData[11] = finalDataLen >> 24;

But I couldn't find any info on the structure of the data that ends up being compressed. How does is the actual pixel data, wall data, etc arranged (prior to compression of course)?

bumpp