有没有办法让 Java 将 PNG 读取为 TYPE_INT_RGB / TYPE_INT_ARGB？

Question

When I read a PNG image in Java using javax.imageio.ImageIO.read() , the resulting BufferedImage is of TYPE_3BYTE_BGR or TYPE_4BYTE_ABGR depending on transparency.

I'm processing very large images (64+ megapixels), and need them in TYPE_INT_RGB / TYPE_INT_ARGB format, which requires an expensive and very-large-chunk-of-memory hogging repainting of the image onto a new image in the correct format, which is causing OOMs.

It would be much better if I could somehow persuade ImageIO to read the image in the desired format from the get-go - is there any way of doing that? Thanks!

Answer 1

Yes, it is possible to read into a predefined type of BufferedImage , given that the type is supported by and compatible with the reader plugin. Most often the TYPE_#BYTE_* types are compatible with the TYPE_INT_* types, and this is the case for the standard PNGImageReader .

To make it work, you need access to the ImageReader directly, and use the read method that takes an ImageReadParam to control the type of image. It's possible to read into a pre-allocated image by using the ImageReadParam.setDestination(..) method, or to just specify the type of image and let the reader plugin allocate it for you by using ImageReadParam.setDestinationType(..) like I will show below.

Here's a short stand-alone code sample that shows how to read into a specific image type:

public static void main(String[] args) throws IOException {
    File input = new File(args[0]);

    try (ImageInputStream stream = ImageIO.createImageInputStream(input)) {
        // Find a suitable reader
        Iterator<ImageReader> readers = ImageIO.getImageReaders(stream);
        if (!readers.hasNext()) {
            throw new IIOException("No reader for " + input);
        }

        ImageReader reader = readers.next();
        try {
            reader.setInput(stream);

            // Query the reader for types and select the best match
            ImageTypeSpecifier intPackedType = getIntPackedType(reader);
            System.out.println("intPackedType = " + intPackedType);

            // Pass the type to the reader using read param
            ImageReadParam param = reader.getDefaultReadParam();
            param.setDestinationType(intPackedType);

            // Finally read the image
            BufferedImage image = reader.read(0, param);
            System.out.println("image = " + image);
        }
        finally {
            reader.dispose();
        }
    }
}

private static ImageTypeSpecifier getIntPackedType(ImageReader reader) throws IOException {
    Iterator<ImageTypeSpecifier> types = reader.getImageTypes(0);

    while (types.hasNext()) {
        ImageTypeSpecifier spec = types.next();

        switch (spec.getBufferedImageType()) {
            case BufferedImage.TYPE_INT_RGB:
            case BufferedImage.TYPE_INT_ARGB:
                return spec;
            default:
                // continue searching
        }
    }

    return null;
}

Sample output from one of my runs using a PNG as input:

intPackedType = javax.imageio.ImageTypeSpecifier$Packed@707084ba
image = BufferedImage@45ff54e6: type = 2 DirectColorModel: rmask=ff0000 gmask=ff00 bmask=ff amask=ff000000 IntegerInterleavedRaster: width = 100 height = 100 #Bands = 4 xOff = 0 yOff = 0 dataOffset[0] 0

Where type = 2 means BufferedImage.TYPE_INT_ARGB .