73 lines
2.6 KiB
Java
73 lines
2.6 KiB
Java
package a2geek.apple2.image.encoder.encode;
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import a2geek.apple2.image.encoder.A2Image;
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/**
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* PackBits. The following text is from Wikipedia:
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* <p>
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* PackBits is a fast, simple compression scheme for run-length encoding of data.
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* <p>
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* Apple introduced the PackBits format with the release of MacPaint on the Macintosh computer.
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* This compression scheme is one of the types of compression that can be used in TIFF-files.
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* <p>
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* A PackBits data stream consists of packets of one byte of header followed by data. The header
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* is a signed byte; the data can be signed, unsigned, or packed (such as MacPaint pixels).
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* <p>
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* <table>
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* <tr><th>Header byte</th><th>Data</th></tr>
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* <tr><td>0 to 127</td><td>1 + n literal bytes of data</td></tr>
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* <tr><td>0 to -127</td><td>One byte of data, repeated 1 - n times in the decompressed output</td></tr>
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* <tr><td>-128</td><td>No operation</td></tr>
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* </table>
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* <p>
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* Note that interpreting 0 as positive or negative makes no difference in the output. Runs of
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* two bytes adjacent to non-runs are typically written as literal data.
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* <p>
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* Looking at the bits, setup is:<br/>
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* <table>
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* <tr><td>%0nnn nnnn</td><td>1 + n literal bytes of data</td></tr>
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* <tr><td>%1nnn nnnn</td><td>One byte of data repeated 1 - n times.</td></tr>
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* <tr><td>%1000 0000</td><td>No operation (used to flag END OF FILE; this is a variance)</td></tr>
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* </table>
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*
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* @author a2geek@users.noreply.github.com (sort of)
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* @see http://en.wikipedia.org/wiki/PackBits
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* @see http://developer.apple.com/technotes/tn/tn1023.html
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* @see http://web.archive.org/web/20080705155158/http://developer.apple.com/technotes/tn/tn1023.html
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*/
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public class PackBitsEncoder extends A2Encoder {
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public String getTitle() {
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return "PackBits (Apple)";
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}
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public void encode(A2Image image, int maxSize) {
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byte[] data = image.getBytes();
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encode(data, maxSize);
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}
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public void encode(byte[] data, int maxSize) {
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reset(maxSize);
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for (int i=0; i<data.length; i++) {
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int b = data[i] & 0xff;
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int runSize = getRunSize(data, i, 1, 128);
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if (runSize > 2) {
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// Compress run of repeated bytes:
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addByte(- runSize + 1);
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addByte(b);
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i+= runSize - 1;
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} else {
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// Gather up nonrepeating bytes:
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int count=0;
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for (count=1; i+count<data.length && count<127; count++) {
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b = data[i+count] & 0xff;
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runSize = getRunSize(data, i+count, 1, 128);
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if (runSize > 2) break;
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}
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addByte(count - 1);
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for (int k=0; k<count; k++) {
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addByte(data[i+k] & 0xff);
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}
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i+= count - 1;
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}
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}
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}
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}
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