img2ansi

Block-Based ANSI Art Dithering Algorithm (Brown Dithering Algorithm)

This project implements a unique dithering algorithm specifically designed for converting images into ANSI art. Unlike traditional dithering methods, my approach uses a block-based processing technique optimized for terminal and text-based display.

Key Features

1. Block-Based Processing: Operates on 2x2 pixel blocks instead of individual pixels, allowing for more complex patterns within a single character cell.

2. ANSI Color Quantization: Utilizes a specialized color quantization scheme tailored for the ANSI color palette, ensuring optimal color representation in terminal environments.

3. Unicode Block Character Selection: Chooses the best Unicode block character to represent each 2x2 pixel block, maximizing the detail in the final ANSI art.

4. Dual-Color Representation: Each block is represented by both a foreground and background color, enabling more nuanced color transitions and detail.

5. Edge Detection Integration: Incorporates edge detection to adjust error distribution, preserving important image details.

6. Optimized for Text Output: Designed to produce ANSI escape code sequences, making it ideal for terminal-based image display.

7. Optimized KD Tree Search: Optimized for ANSI art generation by precomputing quantized color distances.

How It Works

The algorithm processes the input image in 2x2 blocks, determining the best Unicode character and color combination to represent each block. It then uses a modified error diffusion technique inspired by Floyd-Steinberg dithering to distribute quantization errors to neighboring blocks.

This approach results in high-quality ANSI art that preserves the detail and color of the original image while optimizing for the constraints of text-based display.

Requirements

Requires OpenCV 4 to be installed.

Example Output

The below examples are 80 column wide images, with a scale factor of 2. The first example uses the default 16-color ANSI palette, while the second example uses the 256 color scheme.

Installation

To build the program, run the following commands:

go build github.com/wbrown/ansi2img/cmd/ansify

Usage

./img2ansi -input <input> [-output <output>] [-width <width>] [-scale <scale>] [-quantization <quantization>] [-maxchars <maxchars>] [-color_method <color_method>] [-palette <palette>] [-kdsearch <kdsearch>] [-cache_threshold <cache_threshold>]

Performance

The following performance options are available. There are tradeoffs between speed and quality. The defaults are chosen to be a good balance between the two. But if you want the absolute best quality, set the -kdsearch option to 0 and the -cache_threshold option to 0. This may cause the program to take multiple minutes to run.

• -kdsearch <int>: Number of nearest neighbors to search in KD-tree, 0 to disable (default 50)

The KD search option is the number of nearest neighbors to search in the KD-tree for the block cache. A value of 0 will disable the KD-tree search and the cache.

• -cache_threshold <float>: Threshold for block cache (default 40)

The block cache is a cache of the block characters that are used to render the image. The cache is used to speed up the program by not having to recompute the blocks for each 2x2 pixel block in the image. It is a fuzzy cache, so it is thresholded on error distance from the target block.

There are built in embedded palettes that have precomputed tables for the colors. These are ansi16, ansi256, and jetbrains32. Each precomputed palette also has three color spaces that are precomputed: RGB, Lab, and Redmean. The default is Redmean.

Colors

By default the program uses the 16-color ANSI palette, split into 8 foreground colors and 8 background colors. There are three palettes built in, selectable by using the -palette option:

• ansi16: The default 16-color ANSI palette
• ansi256: The 256-color ANSI palette
• jetbrains32: The JetBrains color scheme that uses 32 colors by having separate palettes for foreground and background colors. The program performs well without quantization, but if you want to reduce the number of colors in the output, you can use the -quantization option. The default is 256 colors. This isn't the output colors, but the number of colors used in the quantization step.

There are three color space options available: RGB, Lab, and Redmean. The most perceptually accurate is Lab, but it is also the slowest. The default is Redmean.

Image Size

The -width option can be used to set the target width of the output image, this is the primary factor in determining the output ANSI dimensions. The default -scale is 2, which approximately halves the height of the output, to compensate for the fact that characters are taller than they are wide.

  -cache_threshold float
    	Threshold for block cache (default 40)
  -colormethod string
    	Color distance method: RGB, LAB, or Redmean (default "RGB")
  -input string
    	Path to the input image file (required)
  -kdsearch int
    	Number of nearest neighbors to search in KD-tree, 0 to disable (default 50)
  -maxchars int
    	Maximum number of characters in the output (default 1048576)
  -output string
    	Path to save the output (if not specified, prints to stdout)
  -palette string
    	Path to the palette file (Embedded: ansi16, ansi256, jetbrains32) (default "ansi16")
  -quantization int
    	Quantization factor (default 256)
  -scale float
    	Scale factor for the output image (default 2)
  -width int
    	Target width of the output image (default 80)