Use Luminance instead of R and RG for single and dual channel images

jme3-plugins/src/main/java/com/jme3/texture/plugins/StbImageLoader.java

@@ -55,11 +55,11 @@ public Object load(AssetInfo assetInfo) throws IOException {
             if (is16bit || isFloat) {
                 switch (channels) {
                     case 1:
-                        jmeFormat = Image.Format.R16F;
+                        jmeFormat = Image.Format.Luminance16F;
                         desiredChannels = 1;
                         break;
                     case 2:
-                        jmeFormat = Image.Format.RG16F;
+                        jmeFormat = Image.Format.Luminance16FAlpha16F;
                         desiredChannels = 2;
                         break;
                     case 3:

jme3-screenshot-tests/src/test/java/org/jmonkeyengine/screenshottests/texture/TestSingleChannelTexture.java

@@ -0,0 +1,90 @@
+/*
+ * Copyright (c) 2026 jMonkeyEngine
+ * All rights reserved.
+ *
+ * Redistribution and use in source and binary forms, with or without
+ * modification, are permitted provided that the following conditions
+ * are met:
+ *
+ * * Redistributions of source code must retain the above copyright
+ *   notice, this list of conditions and the following disclaimer.
+ *
+ * * Redistributions in binary form must reproduce the above copyright
+ *   notice, this list of conditions and the following disclaimer in the
+ *   documentation and/or other materials provided with the distribution.
+ *
+ * * Neither the name of 'jMonkeyEngine' nor the names of its contributors
+ *   may be used to endorse or promote products derived from this software
+ *   without specific prior written permission.
+ *
+ * THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
+ * "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT
+ * LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR
+ * A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT
+ * OWNER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL,
+ * SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT
+ * LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
+ * DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY
+ * THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
+ * (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
+ * OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
+ */
+package org.jmonkeyengine.screenshottests.texture;
+
+import com.jme3.app.Application;
+import com.jme3.app.SimpleApplication;
+import com.jme3.app.state.BaseAppState;
+import com.jme3.texture.Texture;
+import com.jme3.texture.Texture2D;
+import com.jme3.ui.Picture;
+import org.jmonkeyengine.screenshottests.testframework.ScreenshotTestBase;
+import org.jmonkeyengine.screenshottests.testframework.TestResolution;
+import org.junit.jupiter.api.Test;
+
+/**
+ * Screenshot test for single-channel PNG loading.
+ */
+public class TestSingleChannelTexture extends ScreenshotTestBase {
+
+    private static final int TEXTURE_SIZE = 251;
+
+    @Test
+    public void testSingleChannelTexture() {
+        screenshotTest(new BaseAppState() {
+            @Override
+            protected void initialize(Application app) {
+                SimpleApplication simpleApplication = (SimpleApplication) app;
+
+                attachPicture(simpleApplication, "single-channel-r16",
+                        "Textures/singleChannel/R16.png", 0f);
+                attachPicture(simpleApplication, "single-channel-r8",
+                        "Textures/singleChannel/R8.png", TEXTURE_SIZE);
+            }
+
+            private void attachPicture(SimpleApplication app, String name, String texturePath, float x) {
+                Texture texture = app.getAssetManager().loadTexture(texturePath);
+                Picture picture = new Picture(name);
+                picture.setTexture(app.getAssetManager(), (Texture2D) texture, false);
+                picture.setPosition(x, 0f);
+                picture.setWidth(TEXTURE_SIZE);
+                picture.setHeight(TEXTURE_SIZE);
+                app.getGuiNode().attachChild(picture);
+            }
+
+            @Override
+            protected void cleanup(Application app) {
+            }
+
+            @Override
+            protected void onEnable() {
+            }
+
+            @Override
+            protected void onDisable() {
+            }
+        })
+        .setTestResolution(new TestResolution(TEXTURE_SIZE * 2, TEXTURE_SIZE))
+        .setFramesToTakeScreenshotsOn(1)
+        .run();
+    }
+}