This implementation is a faithful implementation of the ISO/IEC 18004E International Standard with one important exception. Version 1.0 does not support the encoding of 2 byte Shift-JIS character set. This means that two byte Kanji can only be encoded using binary data format which is less efficient than the native Kanji encoding.

The QR code standard is fully described in the ISO/IEC 18004E International Standard and is available for purchase from the ISO Standard Organization. for a nominal fee.

All QR barcode have square layout made up of equal spaced square modules. Depending on the size of the QR code a certain number of finder patterns are included in the code to aid in scanner decoding. The standard specifies 40 versions (sizes) of the QR code from the smallest 21x21 up to 177x177 modules in size.

Examples of two basic QR codes are shown in Figure 27.3. A small sized QR Code (version=2) and Figure 27.4. A medium sized QR code (version=13)

Figure 27.3. A small sized QR Code (version=2)

Figure 27.4. A medium sized QR code (version=13)

Depending on the actual data there are several compaction schema that can be used in order to achieve the greatest possible compression. The standard specifies four different principal schema, Numeric, Alphanumeric, Binary and Kanji.

Depending on the application the user of the library may chose to either select a fixed encodation mode or let the library automatically chose the most efficient encodation method. It is usually best to let the library automatically select a combination of encodation schema that will give the smallest possible symbol size.

The maximum capacity for QR codes dependent on the encodation schema (using the lowest possible error correction overhead) are given in Table 27.1. QR Data capacity

The exact number of characters that can fit in a QR symbol depends on the actual encoding (or compaction) schema used. In short this is used to more efficiently encode ASCII characters to fit more data into a fixed number of bytes. For example if only numeric data is to be encoded then instead of using one byte to hold each digit three digits is stored in 10 bits. Which gives the equivalent capacity that 12 digits takes only 5 bytes to encode.

To encode data into a QR symbol the following principal steps are taken.

The above explanation is by necessity simplified and for those interested into the specific details we refer to the official standard. It is also possible to review the code itself to understand the details.

As mentioned in the previous section the QR standard specifies 40 different sizes of the QR code and the maximum data capacity will also vary depending on the size. Table 27.2. Maximum data capacity for the different symbol sizes in the QR-code. shows the defined sizes and for each size specifies the possible Error correction levels and the maximum data capacity depending on the compaction schema used.

By default the symbol size will be chosen as the smallest possible. However some application require that usage of a fixed size symbol. The symbol size is a parameter of the encodation schema and is adjusted in the creation of the QREncoder.

As shown in Table 27.2. Maximum data capacity for the different symbol sizes in the QR-code. the QR standard specifies four different error correction levels. In the library the error correction can either be set to be chosen automatically or specified manually. The properties of the available error correction levels are given in Table 27.3. QR Error correction levels. The "Error correction capacity" column specifies how large percentage of the codewords that can be destroyed and the code still being decoded.

It is possible that the JpGraph library gives a visually different result than some other available QR encoders. As a matter of fact many QR encoders gives a different visual result from the same input. This does not mean that one QR encoder is more correct than any other. This is a consequence of interpretation of the standard in a way that (without going into technical details) does not in any way affect the decoding of the barcode. It only affects the visual appearance.