Load 1bpp PNG image and get the pixel data as byte array

[mrapavy]

I have a 1bpp PNG image that I would like to load with ImageSharp, rotate it by 90 degrees and then get the pixel data of 1bpp image as byte array so it can be passed to another library which expects it like this.

This is where I am stuck:

            byte[] DecodeImage(byte[] input1bppPng)
            {
                using (var image = Image.Load<???>(input1bppPng))
                {
                    image.Mutate(x => x.Rotate(RotateMode.Rotate90));
                    return MemoryMarshal.AsBytes(image.GetPixelSpan()).ToArray();
                }
            }

[JimBobSquarePants]

What's the actual memory layout of the buffer you require?

Pngs don't actually store the information as 1bpp. A png encoded with a bit depth of 1 means 1 bit per sample or per palette index (not per pixel). Which means the color palette of the indexed png has a maximum of 2 palette entries.

[antonfirsov]

I think the most likely case is that @mrapavy needs a bit array from a very compact PNG (not an expert on this but yeah, probably palettized I guess).

In this case what's needed is Image.Load<L8> to get the image data as 1 / 0 bytes, then a manual compression to a byte[] interpreted as a bit-array. We don't have any built-in helpers to achieve this second step.

@mrapavy does this help?

[JimBobSquarePants]

There's also A8 which gaming engines use for 8 bit textures.

[mrapavy]

I think the most likely case is that @mrapavy needs a bit array from a very compact PNG (not an expert on this but yeah, probably palettized I guess).

In this case what's needed is Image.Load<L8> to get the image data as 1 / 0 bytes, then a manual compression to a byte[] interpreted as a bit-array. We don't have any built-in helpers to achieve this second step.

@mrapavy does this help?

It does, thank you.

[mrapavy]

What's the actual memory layout of the buffer you require?

Pngs don't actually store the information as 1bpp. A png encoded with a bit depth of 1 means 1 bit per sample or per palette index (not per pixel). Which means the color palette of the indexed png has a maximum of 2 palette entries.

The actual required memory layout is bit array (with palette of two colors). The comment posted by @antonfirsov helps.

[antonfirsov]

In this case what's needed is Image.Load<L8> to get the image data as 1 / 0 bytes

@mrapavy this will be rather 255 / 0 I guess