ADDITIONAL SYSTEM INFORMATION :
It occurs both on Java 8 and Java 23

A DESCRIPTION OF THE PROBLEM :
While an 8-bit greyscale image is recognized correctly, a 4-bit greyscale image (created using: magick test.png -depth 4 -colorspace gray -define png:color-type=0 -define png:bit-depth=4 test.grey.converted-4bit.png)
is recognized as RGB.


STEPS TO FOLLOW TO REPRODUCE THE PROBLEM :
1. Create a 4-bit grey scale image using Image Magick (starting with an arbitrary color test.png):
magick test.png -depth 4 -colorspace gray -define png:color-type=0 -define png:bit-depth=4 test.grey.converted-4bit.png

2. Execute the code provided in Test Case Code field below.
It shows that ImageReader.getRawImageType(...).getColorModel().getColorSpace().getType() returned 5 (=RGB) instead of the expected 6 (=GRAY).


EXPECTED VERSUS ACTUAL BEHAVIOR :
EXPECTED -
17:58:15.942 [main] DEBUG com.mycompany.colorspace.test.ColorSpaceRecognition - ImageIO.createImageInputStream(/test.grey.converted-4bit.png)
17:58:16.034 [main] DEBUG com.mycompany.colorspace.test.ColorSpaceRecognition - rawImageType.getColorModel().getColorSpace().getType(): 6 (5=RGB, 6=GRAY, 3=YCbCr)
...
ACTUAL -
17:58:15.942 [main] DEBUG com.mycompany.colorspace.test.ColorSpaceRecognition - ImageIO.createImageInputStream(/test.grey.converted-4bit.png)
17:58:16.034 [main] DEBUG com.mycompany.colorspace.test.ColorSpaceRecognition - rawImageType.getColorModel().getColorSpace().getType(): 5 (5=RGB, 6=GRAY, 3=YCbCr)
...

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ColorSpaceRecognition.java:
package com.mycompany.colorspace.test;

import com.google.common.io.ByteStreams;
import java.awt.color.ColorSpace;
import java.awt.image.BufferedImage;
import java.io.ByteArrayInputStream;
import java.io.IOException;
import java.io.InputStream;
import java.util.Iterator;
import javax.imageio.ImageIO;
import javax.imageio.ImageReader;
import javax.imageio.ImageTypeSpecifier;
import javax.imageio.stream.ImageInputStream;
import org.slf4j.Logger;
import org.slf4j.LoggerFactory;

public class ColorSpaceRecognition {

    private static final Logger logger = LoggerFactory.getLogger(ColorSpaceRecognition.class);

    public static void main(String[] args) throws IOException {
        // exported from Gimp as grey 8bit, recognized correctly as Grey
        // printColorSpaceInfos("/test.grey.8bit.png");

        // magick test.png -depth 4 -colorspace gray -define png:color-type=0 -define png:bit-depth=4 test.grey.converted-4bit.png
        printColorSpaceInfos("/test.grey.converted-4bit.png");
    }

    private static void printColorSpaceInfos(String resourceName) throws IOException {
        printRawColorSpace(resourceName);
        printNonRawColorSpace(resourceName);
    }

    private static void printNonRawColorSpace(String resourceName) throws IOException {
        logger.debug("ImageIO.read({})", resourceName);
        BufferedImage image = ImageIO.read(new ByteArrayInputStream(getResourceAsBytes(resourceName)));
        if (image == null) {
            throw new RuntimeException("image format not supported");
        }
        logger.debug(" image.getColorModel().getColorSpace().getType(): {} ({}=RGB, {}=GRAY, {}=YCbCr)",
                image.getColorModel().getColorSpace().getType(), ColorSpace.TYPE_RGB, ColorSpace.TYPE_GRAY, ColorSpace.TYPE_YCbCr);

        logger.debug(" image.getType(): {}", image.getType());
        logger.debug(" image.getColorModel().getPixelSize(): {}",
                image.getColorModel().getPixelSize());
        logger.debug(" image.getColorModel().getNumColorComponents(): {}",
                image.getColorModel().getNumColorComponents());
        logger.debug(" image.getColorModel().getComponentSize().length: {}",
                image.getColorModel().getComponentSize().length);
        for (int i = 0; i < image.getColorModel().getComponentSize().length; i++) {
            logger.debug(" image.getColorModel().getComponentSize()[{}]: {}", i,
                    image.getColorModel().getComponentSize()[i]);
        }
        logger.debug(" image.getColorModel().getColorSpace().isCS_sRGB(): {}",
                image.getColorModel().getColorSpace().isCS_sRGB());
        logger.debug(" image.getSampleModel().getNumBands(): {}",
                image.getSampleModel().getNumBands());
    }

    private static void printRawColorSpace(String resourceName) throws IOException {

        logger.debug("ImageIO.createImageInputStream({})", resourceName);
        try (ImageInputStream input
                = ImageIO.createImageInputStream(new ByteArrayInputStream(getResourceAsBytes(resourceName)))) {
            Iterator<ImageReader> readers = ImageIO.getImageReaders(input);
            if (!readers.hasNext()) {
                throw new RuntimeException("image format not supported");
            } else {
                ImageReader reader = readers.next();

                try {
                    reader.setInput(input);

                    ImageTypeSpecifier rawImageType = reader.getRawImageType(0);
                    if (rawImageType == null) {
                        logger.debug(" rawImageType: null");
                    } else {
                        logger.debug(" rawImageType.getColorModel().getColorSpace().getType(): {} ({}=RGB, {}=GRAY, {}=YCbCr)",
                                rawImageType.getColorModel().getColorSpace().getType(), ColorSpace.TYPE_RGB, ColorSpace.TYPE_GRAY, ColorSpace.TYPE_YCbCr);
                        logger.debug(" rawImageType.getColorModel().getPixelSize(): {}",
                                rawImageType.getColorModel().getPixelSize());
                        logger.debug(" rawImageType.getColorModel().getNumColorComponents(): {}",
                                rawImageType.getColorModel().getNumColorComponents());
                        logger.debug(" rawImageType.getColorModel().getComponentSize().length: {}",
                                rawImageType.getColorModel().getComponentSize().length);
                        for (int i = 0; i < rawImageType.getColorModel().getComponentSize().length; i++) {
                            logger.debug(" rawImageType.getColorModel().getComponentSize()[{}]: {}", i,
                                    rawImageType.getColorModel().getComponentSize()[i]);
                        }
                        logger.debug(" rawImageType.getColorModel().getColorSpace().isCS_sRGB(): {}",
                                rawImageType.getColorModel().getColorSpace().isCS_sRGB());
                    }
                } finally {
                    reader.dispose();
                }
            }
        }
    }

    private static byte[] getResourceAsBytes(String resourceName) {
        return getResourceAsBytes(resourceName, ColorSpaceRecognition.class);
    }

    private static byte[] getResourceAsBytes(String resourceName, Class<?> clazz) {
        try (InputStream in = clazz.getResourceAsStream(resourceName)) {
            if (in == null) {
                throw new RuntimeException("Resource '" + resourceName
                        + "' could not be loaded with ClassLoader for " + clazz.getCanonicalName());
            }
            return ByteStreams.toByteArray(in);
        } catch (IOException ex) {
            throw new RuntimeException(ex);
        }
    }
}


pom.xml:
<?xml version="1.0" encoding="UTF-8"?>
<project xmlns="http://maven.apache.org/POM/4.0.0" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:schemaLocation="http://maven.apache.org/POM/4.0.0 http://maven.apache.org/xsd/maven-4.0.0.xsd">
    <modelVersion>4.0.0</modelVersion>
    <groupId>com.mycompany</groupId>
    <artifactId>colorspace-test-greyscale</artifactId>
    <version>1.0-SNAPSHOT</version>
    <packaging>jar</packaging>
    <name>colorspace-test-greyscale</name>
    <properties>
        <project.build.sourceEncoding>UTF-8</project.build.sourceEncoding>
        <maven.compiler.source>1.8</maven.compiler.source>
        <maven.compiler.target>1.8</maven.compiler.target>
        <exec.mainClass>com.mycompany.colorspace.test.ColorSpaceRecognition</exec.mainClass>
    </properties>
    <dependencies>
        <dependency>
            <groupId>ch.qos.logback</groupId>
            <artifactId>logback-classic</artifactId>
            <version>1.2.8</version>
        </dependency>
        <dependency>
            <groupId>com.google.guava</groupId>
            <artifactId>guava</artifactId>
            <version>31.0.1-jre</version>
        </dependency>
    </dependencies>
</project>
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