go-qrcode

The encoding process transforms Unicode Kanji characters into a compressed 13-bit representation using their Shift JIS values.

Encoding Steps

1. Convert: Transform the Unicode character to its 2-byte Shift JIS representation.
2. Adjust: Apply an adjustment formula based on the Shift JIS range.
3. Split: Separate the adjusted value into high and low bytes.
4. Encode: Calculate the final 13-bit value using the formula: (high × 0xC0) + low.

Mathematical Formula

Given a Shift JIS code code (2 bytes):

// Step 1: Adjust the base
if (code >= 0x8140 && code <= 0x9FFC) {
    adjusted = code - 0x8140
} else if (code >= 0xE040 && code <= 0xEBBF) {
    adjusted = code - 0xC140
}

// Step 2: Split into high and low bytes
high = adjusted >> 8      // Upper byte
low = adjusted & 0xFF     // Lower byte

// Step 3: Calculate encoded value (13-bit result)
encoded = (high * 0xC0) + low

When running go-qrcode in a WebAssembly environment, you can use the generateQRCode function to create QR codes. The function accepts a content string and an options object, returning a JSON object containing the success status, any error messages, and the resulting image as a Base64 encoded string.

Options Object Schema

let option = {
	encodeVersion: 0,   // 0 - 40
	encodeMode: 2,      // 0 - 3
	encodeECLevel: "Q", // L, M, Q, H
	
	outputBgColor: "#123123",   // #000000 - #ffffff
	outputBgTransparent: false, // true - false
	outputQrColor: "#666666",   // #000000 - #ffffff
	outputQrWidth: 20,          // 0 - 256
	outputCircleShape: true,    // true - false
	outputImageEncoder: "png",  // png, jpeg, jpg
	outputMargin: 20,           // 0 - 256
}
let r = generateQRCode("content", option)
// output:
// {
//     "success": true,
//     "error": "",
//     "base64EncodedImage": "iVBORw0KGgoAAAANSUhEUgAAAmwAAAJ... more"
// }

To determine if a character can be encoded using Kanji mode, it must satisfy several criteria. This is particularly important when deciding whether to use Kanji mode or fall back to byte mode (UTF-8).

Detection Criteria

1. Kanji Check: The character must be in the Japanese Kanji Unicode ranges (e.g., CJK Unified Ideographs U+4E00-U+9FFF).
2. Shift JIS Conversion: The character must successfully convert to a 2-byte Shift JIS representation.
3. Range Validation: The resulting Shift JIS code must fall within:
   • 0x8140 to 0x9FFC OR
   • 0xE040 to 0xEBBF

Automatic Mode Selection Logic

Use Kanji mode only if ALL characters in the data are valid Kanji characters that meet the criteria above. If any character fails validation (e.g., Hiragana, Katakana, or rare Kanji outside the ranges), you must fall back to a compatible mode like byte mode with UTF-8.

Implementation Logic

IsKanji(character) {
    shiftJIS = UnicodeToShiftJIS(character)

    if (shiftJIS.length != 2) {
        return false
    }

    code = (shiftJIS[0] << 8) | shiftJIS[1]

    return (code >= 0x8140 && code <= 0x9FFC) ||
           (code >= 0xE040 && code <= 0xEBBF)
}

You can implement the qrcode.Writer interface to create custom output formats (e.g., specific image formats or network streams).

Logic for rendering

When implementing Write(mat matrix.Matrix), you should iterate through the 2D array of matrix.State. You can determine the color of a pixel/block by checking the state of each cell.

Generally, states that represent 'set' data (like StateTrue, StateInit, StateVersion, StateFormat, or StateFinder) are treated as the foreground color (e.g., BLACK), while StateFalse or ZERO are treated as the background (e.g., WHITE).

Implementation Pseudocode

// define your own writer structure to implement `Writer` interface.
object writer {};

writer.Write(matrix.Matrix):
	// these should be `IMAGE` stream controller, it receives matrix
	object paint;  
	
	// set use BLACK, unset use WHITE. Or provides matrix.State to colors mapping
	// so that you can control QR Image output intensively. 
	foreach row in matrix:
		foreach column in row:
			if column.State in [StateFalse]:
				// paint a WHITE square block
				paint.draw(x, y, WHITE);
			else:
				// paint a BLACK square block
				paint.draw(x, y, BLACK);
	// loop end;
	// output paint;