How to use .png() to save a greyscale (1 channel), 1 bit image?

[amitm02]

I'm trying to use the .png() function to save:

• An image to greyscale (1 channel), 1-bit png.
• An image to greyscale (1 channel), 2-bit png.

I couldn't understand from the docs what should be the right "options" parameters.
appreciate any help.
awsome library BTW.

[lovell]

Convert to greyscale and save as 1-bit palette PNG:

const png1 = await sharp(input)
  .greyscale()
  .png({ colours: 2 })
  .toBuffer()

Convert to greyscale and save as as 2-bit palette PNG:

const png2 = await sharp(input)
  .greyscale()
  .png({ colours: 4 })
  .toBuffer()

[amitm02]

am i to understand from link
By default, PNG output is full colour at 8 or 16 bits per pixel. Indexed PNG input at 1, 2 or 4 bits per pixel is converted to 8 bits per pixel.
That selecting .png({ colours: 2 }) will not really reduce the file size?

[lovell]

Set palette to true for slower, indexed PNG output.

options.colours maximum number of palette entries, sets palette to true (optional, default 256)

When you provide a value for colours, you are changing from the default behaviour of full colour output to instead using indexed colour output, which usually reduces the file size.

[amitm02]

I get a smaller file with

{
            const optimizedBuffer = await sharp(buffer)
            .greyscale()
            .resize(595)
            .png({quality: 30})
            .toBuffer();
        }

than

 const optimizedBuffer = await sharp(buffer)
            .greyscale()
            .resize(595)
            .png({colors: 2})
            .toBuffer();  

if (assumingly) in PNG, quality just changes the palette,
How can it be? isn't 1bpp should give the smallest image size?

[lovell]

Dropping the colours to 2 may increase the amount of dithering, which can increase entropy and reduce the LZW compression ratio. Dropping quality allows the quantisation to throw detail away. You'll need to experiment for a given input, perhaps also setting dither to zero.

[amitm02]

setting dither to zero had no effect on image size

[lovell]

Is this issue superseded by #2855?

[lovell]

I hope this information helped. I'll close as it looks like #2855 supersede this, but please feel free to re-open with more details if further assistance is required.