The Floyd-Steinberg dithering strategy fails to produce a dithered output when the target palette is 1-bit (black & white). Instead of the expected error diffusion pattern, the resulting image shows a hard threshold "cut-off" effect, where gray areas are converted to solid black or white blocks.

Example

Screenshot of original dashboard:

Undithered 1-Bit PNG:

Dithered 1-Bit PNG (suggested fix applied):

Steps to Reproduce

Select Floyd-Steinberg as the dithering method.

Select a 1-bit or 2-color grayscale palette.

Observe the output image: it lacks the characteristic "dotted" appearance of dithering in transition areas (gradients or light gray backgrounds).

Root Cause Analysis

The issue lies in the FloydSteinbergStrategy.ts implementation. For the colors === 2 case, the code uses either .out('-monochrome') or .out('-type', 'Bilevel') immediately after the dither command.

In ImageMagick, both -monochrome and -type Bilevel are "high-level" operators that often force a simple 50% thresholding algorithm, effectively discarding the error diffusion matrix calculated by the preceding -dither FloydSteinberg command.

Suggested Fix

The dithering should be handled by -colors 2 while keeping the image in a flexible colorspace during processing. The final bit-depth reduction is already handled by the configureOutputFormat utility later in the pipeline, so forcing Bilevel inside the strategy is counterproductive.

Modified Code in FloydSteinbergStrategy.ts:

      if (colors === 2) {
        // Binary (black & white) with error diffusion
        return image.out('-dither', 'FloydSteinberg').out('-colors', '2')  // instead of image.out('-dither', 'FloydSteinberg').out('-monochrome')
      } else if (colors !== undefined && colors > 2) {
        // Multi-level grayscale with error diffusion
        return image
          .out('-dither', 'FloydSteinberg')
          .out('-colors', String(colors))
      }