Converting an OpenCV mat to a JavaFX image not working when using ByteBuffer and PixelFormat [closed]

Problem

You mention your test Mats have 3 channels. The JavaFX PixelBuffer class expects one of the two following pixel formats:

• BGRA premultiplied (when the pixels are in a ByteBuffer)

• ARGB premultiplied (when the pixels are in an IntBuffer)¹

Both those formats have 4 channels. The PixelBuffer thinks your buffer is too small because it only has 3 channels. Presumably your data is missing the alpha channel.

PixelBuffer

The primary benefit of PixelBuffer is the ability to use direct NIO Buffers². Direct buffers hold their data in native memory (i.e., off the Java heap). You can put such buffers in a PixelBuffer without copying the data from native memory into Java memory. A WritableImage can then be used to display the image from that data. This is the most efficient solution if your image data is in native memory and should stay there³. But it does require you to be able to get a direct buffer to the native memory to be fully effective.

If you are not using Java 22+ then there's no public Java API to get a direct ByteBuffer for a specific address. You'd either have to already have the direct ByteBuffer, make use of internal JDK APIs (if there are any), or use NewDirectByteBuffer from the Java Native Interface API (which requires writing your own native code). There may be a third-party library that can help with this (e.g., maybe JNA, not sure).

If you are using Java 22+ and you know the memory address of the data then you can get a direct ByteBuffer to the data via a MemorySegment⁴:

// import java.lang.foreign.MemorySegment;
// import javafx.scene.image.Image;
// import javafx.scene.image.PixelBuffer;
// import javafx.scene.image.PixelFormat;
// import javafx.scene.image.WritableImage;

public Image fromAddress(long address, int width, int height) {
  int length = width * height * 4; // may want to guard against overflow
  var segment = MemorySegment.ofAddress(address).reinterpret(length);

  var buffer = segment.asByteBuffer();
  var format = PixelFormat.getByteBgraPreInstance();
  var pixels = new PixelBuffer<>(width, height, buffer, format);

  return new WritableImage(pixels);
}

Unfortunately, if you can't get a direct ByteBuffer to the data then you'll have to copy the data into a Java byte[] somehow. You can then wrap the byte[] in a ByteBuffer and still use PixelBuffer. This will at least limit the number of copies to one. And this is what you seem to be currently trying to do, it's just you have the wrong number of channels.

PixelWriter

However, PixelBuffer rather limits the pixel formats you can use. If you want to use any of the pixel formats provided by the JavaFX API, then you can use the PixelWriter of a WritableImage.

// import java.nio.Buffer;
// import javafx.scene.image.Image;
// import javafx.scene.image.PixelFormat;
// import javafx.scene.image.WritableImage;

public <T extends Buffer> Image fromBuffer(
    T buffer, int width, int height, PixelFormat<T> format) {
  var image = new WritableImage(width, height);

  // This assumes the pixel format has 4 channels when computing the
  // value for 'scanlineStride'. Use a different value, or even try
  // to dynamically compute the value, as appropriate.
  int scanlineStride =
      switch (buffer) {
        case ByteBuffer _ -> width * 4;
        case IntBuffer _ -> width;
        default -> throw new IllegalArgumentException("unsupported buffer type");
      };

  var writer = image.getPixelWriter();
  writer.setPixels(0, 0, width, height, format, buffer, scanlineStride);

  return image;
}

Be aware that the setPixels methods copy the data from the buffer. Therefore if you copied the data from native memory to Java memory already, then this approach will result in copying the data twice. But if you can get a direct ByteBuffer to the data then this approach will only copy the data once while also giving you more freedom regarding the pixel format.

General Solution

So, you have two general solutions within the JavaFX API:

1. Modify your pixel data to have 4 channels, whether in Java or via OpenCV, then continue using PixelBuffer. Make sure the pixel format is premultiplied BGRA (assuming byte buffer).

2. Switch from PixelBuffer to PixelWriter with an appropriate 3-channel PixelFormat.

In all approaches mentioned in this answer, the pixel data must be the raw uncompressed pixel data. The format used to read the data is determined by the PixelFormat.

^{1. Note you can use ByteBuffer#asIntBuffer() to decorate a ByteBuffer with an IntBuffer. This would let you use the ARGB premultiplied format even with a direct ByteBuffer, assuming the underlying bytes are in that format. Make sure you configure the correct byte order.}

^{2. While PixelBuffer uses NIO Buffers, it is not itself an NIO buffer.}

^{3. As noted, a PixelBuffer that's using a ByteBuffer requires the pixel data bytes to be in BGRA format. That's a 4-channel format. If your data is currently in a 3-channel format, such as BGR, then note converting to BGRA will likely result in the image's memory footprint increasing by approximately 33%.}

^{4. The MemorySegment interface is part of the Foreign Function & Memory API added to the standard library in Java 22.}