/// <summary>
/// Reverses the filtering process done on the decompressed
/// datastream to get the unfiltered datastream.
/// </summary>
/// <param name="filter_type">The type of filtering process done on the section of data.</param>
private byte[] unfilter_scanline(Filter_Types filter_type, byte[] filtered_scanline_bytes, byte[] previous_scanline_bytes)
{
byte[] result = null;
switch (filter_type)
{
case (Filter_Types.SUB):
result = unfilter_sub_scanline(filtered_scanline_bytes);
break;
case (Filter_Types.UP):
result = unfilter_up_scanline(filtered_scanline_bytes, previous_scanline_bytes);
break;
case (Filter_Types.AVERAGE):
result = unfilter_avg_scanline(filtered_scanline_bytes, previous_scanline_bytes);
break;
case (Filter_Types.PAETH):
result = unfilter_paeth_scanline(filtered_scanline_bytes, previous_scanline_bytes);
break;
default:
result = new byte[filtered_scanline_bytes.Length - 1];
for (int i = 1; i < filtered_scanline_bytes.Length; i++)
{
result[i - 1] = filtered_scanline_bytes[i];
}
break;
}
return(result);
}
/// <summary>
/// Reverses the filtering of the bytes passed in.
/// Assumes that the bytes are a one-dimensional array of all of
/// the bytes of data. This means that one scanline comes after
/// another, and this method uses the stored height/width fields
/// of the PNG file decoded to determine when a new scanline is
/// reached. Also assumes that the PNG file is valid.
/// </summary>
/// <param name="decompressed_datastream">The decompressed datastream to be unfiltered.</param>
/// <returns>An array of bytes that contains the unfiltered, decompressed datastream's bytes.</returns>
private byte[] unfilter_datastream(byte[] decompressed_datastream)
{
if (!is_valid_png ||
decompressed_datastream == null)
{
return(null);
}
List <byte> unfiltered_bytes = new List <byte>();
// process the first scanline
byte[] previous_scanline = new byte[width_p * pixel_width_p + 1];
byte[] current_scanline = new byte[width_p * pixel_width_p + 1];
int byte_index = 0;
Filter_Types filter_type = (Filter_Types)decompressed_datastream[byte_index++];
current_scanline[0] = decompressed_datastream_p[0];
// specially handle the first scanline (no previous scanline) and add results to
// the unfiltered bytes list
for (; byte_index < width_p * pixel_width_p + 1;)
{
current_scanline[byte_index] = decompressed_datastream[byte_index];
++byte_index;
}
byte_index = 0;
previous_scanline = unfilter_scanline(filter_type, current_scanline, previous_scanline);
foreach (byte scanline_byte in previous_scanline)
{
unfiltered_bytes.Add(scanline_byte);
}
// unfilter the other scanlines, and add them to the list
for (int i = 1; i < height_p; i++)
{
int scanline_length = (int)width_p * pixel_width + 1;
int first_bit_index = (scanline_length) * i;
current_scanline[0] = decompressed_datastream[first_bit_index + byte_index++];
filter_type = (Filter_Types)current_scanline[0];
for (; byte_index < scanline_length;)
{
current_scanline[byte_index] = decompressed_datastream[first_bit_index + byte_index];
++byte_index;
}
previous_scanline = unfilter_scanline(filter_type, current_scanline, previous_scanline);
foreach (byte scanline_byte in previous_scanline)
{
unfiltered_bytes.Add(scanline_byte);
}
byte_index = 0;
}
return(unfiltered_bytes.ToArray());
}
