Imaging.writeImage ignores the requested PNM format and writes PPM

src/main/java/org/apache/commons/imaging/AbstractImageParser.java

@@ -746,4 +746,24 @@ public final ImageMetadata getMetadata(final File file, final T params) throws I
     public void writeImage(final BufferedImage src, final OutputStream os, final T params) throws ImagingException, IOException {
         throw new ImagingException("This image format (" + getName() + ") cannot be written.");
     }
+
+    /**
+     * Writes the content of a BufferedImage to the specified output stream in the given format.
+     *
+     * <p>
+     * A parser whose {@link #getAcceptedTypes()} lists more than one format overrides this method so that the format selects the output; the rest write their
+     * only format and ignore the argument.
+     * </p>
+     *
+     * @param src    An image giving the source content for output.
+     * @param os     A valid output stream for storing the formatted image.
+     * @param format The format the caller asked for; one of {@link #getAcceptedTypes()}.
+     * @throws ImagingException In the event that the output format cannot handle the input image.
+     * @throws IOException      In the event of a write error from the output stream.
+     *
+     * @since 1.0.0-alpha7
+     */
+    protected void writeImageForFormat(final BufferedImage src, final OutputStream os, final ImageFormat format) throws ImagingException, IOException {
+        writeImage(src, os, null);
+    }
 }

src/main/java/org/apache/commons/imaging/Imaging.java

@@ -917,7 +917,7 @@ public static void writeImage(final BufferedImage src, final OutputStream output
         Objects.requireNonNull(format, "format");
 
         final AbstractImageParser<?> imageParser = ImageParserFactory.getImageParser(format);
-        imageParser.writeImage(src, outputStream, null);
+        imageParser.writeImageForFormat(src, outputStream, format);
     }
 
     /**

src/main/java/org/apache/commons/imaging/formats/pnm/PnmImageParser.java

@@ -343,4 +343,13 @@ public void writeImage(final BufferedImage src, final OutputStream os, final Pnm
 
         writer.writeImage(src, os, params);
     }
+
+    @Override
+    protected void writeImageForFormat(final BufferedImage src, final OutputStream os, final ImageFormat format) throws ImagingException, IOException {
+        final PnmImagingParameters params = new PnmImagingParameters();
+        if (format instanceof ImageFormats) {
+            params.setSubtype((ImageFormats) format);
+        }
+        writeImage(src, os, params);
+    }
 }

src/test/java/org/apache/commons/imaging/formats/pnm/PnmImageParserTest.java

@@ -22,6 +22,7 @@
 
 import java.awt.image.BufferedImage;
 import java.awt.image.DataBufferInt;
+import java.io.ByteArrayOutputStream;
 import java.io.IOException;
 import java.nio.charset.Charset;
 import java.nio.charset.StandardCharsets;
@@ -36,6 +37,18 @@ class PnmImageParserTest {
 
     private static final Charset US_ASCII = StandardCharsets.US_ASCII;
 
+    @Test
+    void testWriteImageHonorsRequestedFormat() throws ImagingException, IOException {
+        final BufferedImage image = new BufferedImage(2, 2, BufferedImage.TYPE_INT_RGB);
+        final ImageFormats[] formats = { ImageFormats.PBM, ImageFormats.PGM, ImageFormats.PPM, ImageFormats.PAM };
+        final String[] magic = { "P4", "P5", "P6", "P7" };
+        for (int i = 0; i < formats.length; i++) {
+            final ByteArrayOutputStream out = new ByteArrayOutputStream();
+            Imaging.writeImage(image, out, formats[i]);
+            assertEquals(magic[i], new String(out.toByteArray(), 0, 2, US_ASCII), formats[i].name());
+        }
+    }
+
     @Test
     void testGetImageInfo_happyCase() throws ImagingException, IOException {
         final byte[] bytes = "P1\n3 2\n0 1 0\n1 0 1\n".getBytes(US_ASCII);