Either I do not understand how to create a PixelFormat of Type.BYTE_INDEXED and use it to create an indexed grayscale image (with values ranging from 0..255), or there is a problem in PixelFormat.createByteIndexedInstance(colors). I suspect something is awry in dealing with bytes as unsigned values. Documentation and sample code on these classes is a bit thin.

This test app:

import javafx.application.Application;
import javafx.scene.Scene;
import javafx.scene.image.ImageView;
import javafx.scene.image.PixelFormat;
import javafx.scene.image.WritableImage;
import javafx.scene.layout.StackPane;
import javafx.stage.Stage;

public class ByteIndexedTest extends Application
{
  @Override public void start(Stage primaryStage)
  {
    // create an color table of ARGB colors with r == g == b
    int[] colors = new int[256];
    for (int i = 0; i < 256; i++)
    {
      colors[i] = (255 << 24) | (i << 16) | (i << 8) | i;
    }

    // create a byte-indexed PixelFormat with the color table
    PixelFormat pixelFormat = PixelFormat.createByteIndexedInstance(colors);

    int width = 256;
    int height = 256;

    // create an array of one-byte pixels representing indicies into the color table
    byte[] pixels = new byte[width*height];

    // populate to make a nice gradient dark to light, top to bottom
    for (int y = 0; y < height; y++)
    {
      // simply cast the int to a byte - bytes in PixelFormat.Type.BYTE_INDEXED are interpreted as unsigned
      byte value = (byte)(y & 0xff);

      // all pixels in the current row share the same index into the color table
      for (int x = 0; x < width; x++)
      {
        pixels[y*width+x] = value;
      }
    }

    // create a new WritableImage
    WritableImage image = new WritableImage(width,height);

    // set the pixels using the pixel writer
    image.getPixelWriter().setPixels(0, 0, width, height, pixelFormat, pixels, 0, width);

    StackPane root = new StackPane();
    root.getChildren().add(new ImageView(image));

    Scene scene = new Scene(root);
    primaryStage.setScene(scene);
    primaryStage.show();
  }


throws this exception:

Caused by: java.lang.NullPointerException
at com.sun.javafx.image.impl.BaseByteToByteConverter.<init>(BaseByteToByteConverter.java:45)
at com.sun.javafx.image.impl.General$ByteToByteGeneralConverter.<init>(General.java:69)
at com.sun.javafx.image.impl.General.create(General.java:44)
at com.sun.javafx.image.PixelUtils.getB2BConverter(PixelUtils.java:223)
at com.sun.prism.Image$ByteAccess.setPixels(Image.java:770)
at com.sun.prism.Image.setPixels(Image.java:606)
at javafx.scene.image.WritableImage$2.setPixels(WritableImage.java:199)
at ByteIndexedTest.start(ByteIndexedTest.java:46)