Crate plot_icon

Overview

This is a lib crate for generating standard 19-circle icons in png and in svg format.

Output is Vec<u8> png data, or svg::Document with svg data, both could be easily printed into files.

The identicon color scheme and elements arrangement follow the published javascript code for polkadot identicon generation. This crate is intended mainly for use by Signer.

Input

Identicon is generated for &[u8] input slice. During identicon generation, this input slice gets hashed, therefore, any length would be acceptable.

Typical input slice is a public key. Public key is often encountered as a hexadecimal string (d43593c715fdd31c61141abd04a99fd6822c8558854ccde39a5684e7a56da27d) or as a base58 network-specific string (5GrwvaEF5zXb26Fz9rcQpDWS57CtERHpNehXCPcNoHGKutQY), both could be easily transformed into &[u8] input.

Crate also supports generation of identicon-like images with user-provided colors in RGBA format.

PNG

Signer uses images in png format, since svg format is not sufficiently supported on devices side and might be non-deterministic. Therefore, this crate sticks mostly to png generation. Feature "pix" (enabled by default) enables generation of png images.

Function generate_png produces png data for identicon, and requires:

• &[u8] slice
• target image size in pixels (u16)
  png images are generated pixel-by-pixel, and the quality of final image is determined by the image size. Each png pixel (with integer coordinates) falling within the identicon circle element (with float circle parameters) gets the color of the circle. Below certain image size (approximately 100 pix) the circles become too pixelated. Also, images with even number of pixels size are off-centered by a pixel.

Signer needs small png identicon icons. Exact parameters are yet TBD (at the moment, identicons are 30 pix and device-independent), however, the straightforward approach with generate_png does not produce acceptable results.

Possible solution is to generate larger identicon and then scale it down in Signer frontend, but it was noticed that the scaling results (pixelation, color distribution) are device-dependent and although a minor thing, it should definitely be avoided in identicon.

To generate reproducible small identicons, the rescaling is performed within the crate. A larger png is generated, and then scaled down to originally desired size. This procedure results in both less pixelated circles and compensated off-centering.

Function generate_png_scaled_custom performs the scaling with custom parameters, and requires:

• &[u8] slice
• target identicon size in pixels (u8 - it is for small identicons, after all)
• scaling factor (u8), how much larger the larger png actually is
• filter (FilterType) used for image resize

The scaling factor reasonable values are in range [4..=8], below it the pixelation persists, above it the images are not visibly improving anymore, and may even seem blurry.

All filters produce reasonable results, except FilterType::Nearest that yields visibly distorted images and therefore is not recommended.

Function generate_png_scaled_default performs the scaling with default scaling parameters (scaling factor 5 and filter FilterType::Lanczos3) for image with default Signer identicon size (30 pix), and requires only:

• &[u8] slice
  If somehow the generation of the identicon fails, function outputs default-sized (30x30) transparent png image, i.e. it never produces an error.

Function generate_png_with_colors is similar to generate_png, but accepts identicon colors directly, and does not generate color set itself. This is intended mainly for tests. Function generate_png_with_colors requires:

• [[u8; 4]; 19] 19-element set of colors in RGBA format
• target image size in pixels (u16)

Function generate_png_scaled_custom_with_colors is similar to generate_png_scaled_custom, but accepts identicon colors directly, and does not generate color set itself. This is intended mainly for tests. Function generate_png_scaled_custom_with_colors requires:

• [[u8; 4]; 19] 19-element set of colors in RGBA format
• target identicon size in pixels (u8)
• scaling factor (u8)
• filter (FilterType) used for image resize

SVG

Feature "vec" (enabled by default) enables infallible generation of identicon pictures in svg format. Since svg is a vector format, no image size parameters are needed.

Function generate_svg reqires only &[u8] input slice.

Function generate_svg_with_colors uses pre-set colors and is intended mainly for tests. It requires only the color set ([[u8; 4]; 19] 19-element set of colors in RGBA format).

Tests and Examples

Tests in colors.rs module check if the color sets calculated for Alice and Bob are identical to the colors in the corresponding well-known icons.

Doc tests in lib.rs produce various test pics, both png (through different functions and parameters) and svg.

Notes

There are several uncertainties about how the original published code was designed to work, those should be clarified, eventually.

For example, calculated HSL color saturation could range 30..109, and is processed as percents. Crate palette (currently used here) processes saturation values over 100 as percents over 100, and gives some results (slightly different from results for 100% saturation), but it is necessary to check if the calculations in js and here are matching.

See details in code comments.