png comparison

This is the image with PNG_FILTER_NONE:

This is the same image without the call to png_set_filter:

This is the image with the data written at the end, with the PNG_FILTER_NONE switched ON (the colours have changed but the data bit is the same):

Now here is the iamge with the data written at the end with the default libpng behaviour:

The sizes of the compressed bits are exactly the same regardless of whether writing to the end or the beginning.