extracting png files from APNG in java

Question

I have been trying to extract all the png files from an APNG file. i have looked for help and well there isnt much. All i could find is a open source library pngj and with it i am able to get the first frame of a APNG file.

Here is the code i am using

public static void mirror(File orig, File dest, boolean overwrite)
         {
    PngReader pngr = FileHelper.createPngReader(orig);
    PngWriter pngw = FileHelper.createPngWriter(dest, pngr.imgInfo,
            overwrite);
    pngw.setFilterType(FilterType.FILTER_CYCLIC); // just to test all
                                                    // filters
    int copyPolicy = ChunkCopyBehaviour.COPY_ALL;
    pngw.copyChunksFirst(pngr, copyPolicy);
    ImageLine lout = new ImageLine(pngw.imgInfo);
    int cols = pngr.imgInfo.cols;
    int channels = pngr.imgInfo.channels;
    int[] line = new int[cols * channels];
    int aux;
    for (int row = 0; row < pngr.imgInfo.rows; row++) {
        ImageLine l1 = pngr.readRow(row);
        line = l1.unpack(line, false);
        for (int c1 = 0, c2 = cols - 1; c1 < c2; c1++, c2--) {
            for (int i = 0; i < channels; i++) {
                aux = line[c1 * channels + i];
                line[c1 * channels + i] = line[c2 * channels + i];
                line[c2 * channels + i] = aux;
            }
        }
        lout.pack(line, false);
        pngw.writeRow(lout, row);
    }
    pngr.end();
    pngw.copyChunksLast(pngr, copyPolicy);
    pngw.end();
    // // print unknown chunks, just for information
    List<PngChunk> u = ChunkHelper.filterList(pngr.getChunksList()
            .getChunks(), new ChunkPredicate() {
        public boolean match(PngChunk c) {
            return ChunkHelper.isUnknown(c);
        }
    });
    if (!u.isEmpty())
        System.out.println("Unknown chunks:" + u);
}

so basically i am just mirroring an apng file and it gets converted into a png file which is the first frame. So can some one tell me how to get the remaining frames and save them as png files? Any help or hints would be appricated

Answer 1

Actually the PNGJ library (I'm the author) does not support APNG standard (digression: I decided against it because I didn't like APGN approach and because it didn't fit with the idea of my library :loading "huge" data -IDAT- progressively, line by line; and loading the chunks (metadata) in memory directly; APGN abuses the PGN standard by storing frames in chunks).

You could try to use it anyay to do what you want to, but not in an elegant and robust way. Here's an example. Beside being ugly, this would not work for partial frames (frames smaller than the full image, which APNG supports), nor for overlays, nor for paletted images (the later issue would be the easiest to fix) (fixed, hopefully).

This was tested with http://philip.html5.org/tests/apng/028.png

import java.io.File;
import java.io.FileOutputStream;

import ar.com.hjg.pngj.FileHelper;
import ar.com.hjg.pngj.ImageLine;
import ar.com.hjg.pngj.PngHelperInternal;
import ar.com.hjg.pngj.PngReader;
import ar.com.hjg.pngj.PngWriter;
import ar.com.hjg.pngj.chunks.*;

public class ApngSplit {

    private static final String PREFIX = "apngf";

    /** reads a APNG file and tries to split it into its frames */
    public static void process(File orig) throws Exception {
        PngReader pngr = FileHelper.createPngReader(orig);
        File dest = new File(orig.getParent(), PREFIX + "0_" + orig.getName());
        PngWriter pngw = FileHelper.createPngWriter(dest, pngr.imgInfo, true);
        System.out.println("writing default frame " + pngw.getFilename());
        pngr.setChunkLoadBehaviour(ChunkLoadBehaviour.LOAD_CHUNK_ALWAYS);
        pngr.setMaxBytesMetadata(Integer.MAX_VALUE);
        pngr.setMaxTotalBytesRead(Long.MAX_VALUE);
        pngr.setSkipChunkIds(new String[] {});
        int copyPolicy = ChunkCopyBehaviour.COPY_PALETTE | ChunkCopyBehaviour.COPY_ALL_SAFE;
        pngw.copyChunksFirst(pngr, copyPolicy);
        int cols = pngr.imgInfo.cols;
        int channels = pngr.imgInfo.channels;
        for (int row = 0; row < pngr.imgInfo.rows; row++) {
            ImageLine l1 = pngr.readRow(row);
            pngw.writeRow(l1, row);
        }
        pngr.end();
        pngw.copyChunksLast(pngr, copyPolicy);
        pngw.end();
        processExtra2(orig, pngr.getChunksList());
    }

    private static void processExtra2(File orig, ChunksList chunks) throws Exception {
        int numframe = 0;
        FileOutputStream os = null;
        boolean afterIdat = false;
        for (PngChunk chunkApng : chunks.getChunks()) {
            if (chunkApng.id.equals("IDAT"))
                afterIdat = true;
            if (chunkApng.id.equals("fcTL") && afterIdat) {
                numframe++;
                if (os != null)
                    endPng(chunks, os);
                File dest = new File(orig.getParent(), PREFIX + numframe + "_" + orig.getName());
                System.out.println("writing seq " + numframe + " : " + dest);
                os = new FileOutputStream(dest);
                beginPng(chunks, os);
            }
            if (chunkApng.id.equals("fdAT")) {
                ChunkRaw crawf = chunkApng.createRawChunk();
                int seq = PngHelperInternal.readInt4fromBytes(crawf.data, 0);
                ChunkRaw crawi = new ChunkRaw(crawf.len - 4, ChunkHelper.b_IDAT, true);
                System.arraycopy(crawf.data, 4, crawi.data, 0, crawi.data.length);
                crawi.writeChunk(os);
            }
        }
        if (os != null)
            endPng(chunks, os);
    }

    private static void endPng(ChunksList chunks, FileOutputStream fos) throws Exception {
        chunks.getById1(PngChunkIEND.ID).createRawChunk().writeChunk(fos);
        fos.close();
    }

    private static void beginPng(ChunksList chunks, FileOutputStream fos) throws Exception {
        fos.write(new byte[] { -119, 80, 78, 71, 13, 10, 26, 10 }); // signature
        chunks.getById1(PngChunkIHDR.ID).createRawChunk().writeChunk(fos);
        PngChunk plte = chunks.getById1(PngChunkPLTE.ID);
        if (plte != null)
            plte.createRawChunk().writeChunk(fos);
    }

    public static void main(String[] args) throws Exception {
        process(new File("C:/temp/029.png"));
    }

}

Answer 2

Some resources that might help:

1. Those examples show how to read apng files, but it's C and using libpng: https://sourceforge.net/projects/apng/files/libpng/examples/

2. Here's some Java code, but it can only create APNG files, not read them: https://www.reto-hoehener.ch/japng/index.html

3. Here's JavaScript code for reading and displaying APNG files: https://github.com/davidmz/apng-canvas