Pixel order error

[diegotibi]

Hi @EliotJones,
I've found a problem with your lib trying to decode this file:

I was using a simple loop to read pixel by pixel the file but something in the output wasn't correct so I tried to simply copy the image into a new one, with this code:

var stream = File.OpenRead("test.png");
var image = Png.Open(stream);
var streamOut = File.Create("testOut.png");
var builder = PngBuilder.FromPng(image);
builder.Save(streamOut);
streamOut.Close();

The output image is this:

The input file seems to be ok (you can inspect it here)
With other files (created using the same process) this problem doesn't shows up.

Any thoughts?

Thanks

[plaurin]

I've found a bug in RawPngData.cs that will result in the skewing effect seen in the output image above. In my case I had a 4bits per pixel image of width 287, an odd number. On line 48 RawPngData.cs we can see this

var bytesInRow = (1 + (width / pixelsPerByte));

Because pixelsPerByte is 2 (we can fit 2 pixels data of 4bits each in a 8bits byte) we get 1 + 143.5 but because we are working with int the .5 gets trunked. So it is bytesInRow = 1 + 143 = 144 which is obviously wrong because an image of width 286 would give us 1 + (286 / 2) = 144 too!

This means all image with an odd width and less than 16 colors will have this bug.

My dirty solution was to round up the division like this:

var bytesInRow = (int)(1 + Math.Ceiling((double)width / pixelsPerByte));

I hope this could help.