Wikipedia says it does support it.
Raster file formats that support transparency include GIF, PNG, BMP, TIFF, and JPEG 2000, through either a transparent color or an alpha channel.
based on javadoc
https://docs.oracle.com/javase/7/docs/api/javax/imageio/package-summary.html
ImageIO only supports JPEG, PNG, BMP, WBMP, GIF and among them I think only JPEG not supports transparency.
That makes it more complicated though. Could be that they define white/black as “transparent”. Anyway, good to know, I didn’t know BMP supports transparency 
Yeah, and any of this would require a second method else we break all existing code… so we should think about it a lot first.
Just test it with bmp format and it throws this exception:
java.io.IOException: Image can not be encoded with compression type BI_RGB
at java.desktop/com.sun.imageio.plugins.bmp.BMPImageWriter.write(BMPImageWriter.java:322)
at com.jme3.system.JmeDesktopSystem.writeImageFile(JmeDesktopSystem.java:113)
at com.jme3.system.JmeSystem.writeImageFile(JmeSystem.java:134)
at com.overthemoon.main.TestIconGenerator2.savePng(TestIconGenerator2.java:142)
at com.overthemoon.main.TestIconGenerator2.simpleRender(TestIconGenerator2.java:203)
at com.jme3.app.SimpleApplication.update(SimpleApplication.java:271)
at com.jme3.system.lwjgl.LwjglWindow.runLoop(LwjglWindow.java:499)
at com.jme3.system.lwjgl.LwjglWindow.run(LwjglWindow.java:581)
at com.jme3.system.lwjgl.LwjglWindow.create(LwjglWindow.java:423)
at com.jme3.app.LegacyApplication.start(LegacyApplication.java:463)
at com.jme3.app.LegacyApplication.start(LegacyApplication.java:424)
at com.jme3.app.SimpleApplication.start(SimpleApplication.java:125)
at com.overthemoon.main.TestIconGenerator2.main(TestIconGenerator2.java:77)
So below code is not working for bmp.
public void writeImageFile(OutputStream outStream, String format, ByteBuffer imageData, int width, int height) throws IOException {
ImageWriter writer = ImageIO.getImageWritersByFormatName(format).next();
ImageWriteParam writeParam = writer.getDefaultWriteParam();
BufferedImage awtImage;
if (format.equals("jpg")) {
JPEGImageWriteParam jpegParam = (JPEGImageWriteParam) writeParam;
jpegParam.setCompressionMode(ImageWriteParam.MODE_EXPLICIT);
jpegParam.setCompressionQuality(0.95f);
awtImage = new BufferedImage(width, height, BufferedImage.TYPE_INT_BGR);
Screenshots.convertScreenShot2(imageData.asIntBuffer(), awtImage);
awtImage = verticalFlip(awtImage);
} else {
awtImage = new BufferedImage(width, height, BufferedImage.TYPE_4BYTE_ABGR);
Screenshots.convertScreenShot(imageData, awtImage);
}
ImageOutputStream imgOut = new MemoryCacheImageOutputStream(outStream);
writer.setOutput(imgOut);
IIOImage outputImage = new IIOImage(awtImage, null, null);
try {
writer.write(null, outputImage, writeParam);
} finally {
imgOut.close();
writer.dispose();
}
}
Should we treat bmp as non transparent (BufferedImage.TYPE_INT_BGR) ?
Neither it works for wbmp:
java.lang.IllegalArgumentException: Only integral single-band bilevel image is supported.
at java.desktop/com.sun.imageio.plugins.wbmp.WBMPImageWriter.checkSampleModel(WBMPImageWriter.java:311)
at java.desktop/com.sun.imageio.plugins.wbmp.WBMPImageWriter.write(WBMPImageWriter.java:169)
at com.jme3.system.JmeDesktopSystem.writeImageFile(JmeDesktopSystem.java:117)
at com.jme3.system.JmeSystem.writeImageFile(JmeSystem.java:134)
at com.overthemoon.main.TestIconGenerator2.savePng(TestIconGenerator2.java:141)
at com.overthemoon.main.TestIconGenerator2.simpleRender(TestIconGenerator2.java:202)
at com.jme3.app.SimpleApplication.update(SimpleApplication.java:271)
at com.jme3.system.lwjgl.LwjglWindow.runLoop(LwjglWindow.java:499)
at com.jme3.system.lwjgl.LwjglWindow.run(LwjglWindow.java:581)
at com.jme3.system.lwjgl.LwjglWindow.create(LwjglWindow.java:423)
at com.jme3.app.LegacyApplication.start(LegacyApplication.java:463)
at com.jme3.app.LegacyApplication.start(LegacyApplication.java:424)
at com.jme3.app.SimpleApplication.start(SimpleApplication.java:125)
at com.overthemoon.main.TestIconGenerator2.main(TestIconGenerator2.java:76)Ah I just notice this in javadoc of JmeSystem.writeImageFile()
It clearly says to use jpg or png, so I guess we should not care about other formats. 
I’m criticizing the finer details here, but line 119 states:
It indicates a form of compression takes place. BMP is a raw lossless format. So maybe there’s your get-out clause for utilitarian functionality.
Edit: Then again, so is PNG. There’s a discrepancy in the javadoc there, strictly speaking.
I see what you’re saying, but I think it could also be read the other way to say that it takes a raw image (uncompressed pixel stream) and converts it to a standard compressed image format. In any case, it’s a bit ambiguous and it might be a good idea to clarify.
Okay, now main issue with transparent png exporting is fixed on master branch.
https://github.com/jMonkeyEngine/jmonkeyengine/pull/1054
I have yet one small issue, see below generated png icon:
As you can see behind the leaves (marked in red) is transparent.
Any way to fix it ?
And here is my code:
public class IconGeneratorState extends BaseAppState {
static Logger log = LoggerFactory.getLogger(IconGeneratorState.class);
private Node root;
private FrameBuffer offBuffer;
private ViewPort offView;
private Geometry wireBounds;
private boolean debugBounds = false;
private boolean debugTexture = false;
private int width;
private int height;
private File path;
public IconGeneratorState() {
this(256, 256);
}
public IconGeneratorState(int width, int height) {
this.width = width;
this.height = height;
}
@Override
protected void initialize(Application app) {
root = new Node("RootNode:");
Camera offCamera = new Camera(width, height);
offView = getApplication().getRenderManager().createPreView("Offscreen View", offCamera);
offView.setClearFlags(true, true, true);
offView.setBackgroundColor(ColorRGBA.BlackNoAlpha);
// create offscreen framebuffer
offBuffer = new FrameBuffer(width, height, 1);
//setup framebuffer's cam
offCamera.setFrustumPerspective(45f, 1f, 1f, 1000f);
offCamera.setLocation(new Vector3f(0f, 0f, 10f));
offCamera.lookAt(new Vector3f(0f, 0f, 0f), Vector3f.UNIT_Y);
//setup framebuffer's texture
Texture2D offTex = new Texture2D(width, height, Image.Format.RGBA8);
//offTex.setMinFilter(Texture.MinFilter.Trilinear);
//offTex.setMagFilter(Texture.MagFilter.Bilinear);
//setup framebuffer to use texture
offBuffer.setDepthBuffer(Image.Format.Depth);
offBuffer.setColorTexture(offTex);
//set viewport to render to offscreen framebuffer
offView.setOutputFrameBuffer(offBuffer);
// attach the scene to the viewport to be rendered
offView.attachScene(root);
if (debugTexture) {
//setup main scene
Quad mesh = new Quad(2, 2);
Geometry quad = new Geometry("box", mesh);
Material mat = GuiGlobals.getInstance().createMaterial(false).getMaterial();
mat.setTexture("ColorMap", offTex);
//mat.getAdditionalRenderState().setBlendMode(RenderState.BlendMode.Alpha);
quad.setMaterial(mat);
((SimpleApplication) getApplication()).getRootNode().attachChild(quad);
}
//initFilters();
}
private void initFilters() {
FilterPostProcessor fpp = new FilterPostProcessor(getApplication().getAssetManager());
fpp.addFilter(new FXAAFilter());
fpp.addFilter(new ToneMapFilter(Vector3f.UNIT_XYZ.mult(11.0f)));//4.0f
fpp.addFilter(new SSAOFilter(0.5f, 3, 0.2f, 0.2f));
offView.addProcessor(fpp);
}
@Override
protected void cleanup(Application app) {
getApplication().getRenderManager().removePreView(offView);
}
@Override
protected void onEnable() {
offView.setEnabled(true);
}
@Override
protected void onDisable() {
offView.setEnabled(false);
}
float time;
@Override
public void update(float tpf) {
root.updateLogicalState(tpf);
root.updateGeometricState();
updateCamera();
time += tpf;
}
@Override
public void render(RenderManager rm) {
if (path != null && time > 0.2) {
Renderer renderer = rm.getRenderer();
Image image = createFrameBufferImage(offBuffer);
//Texture2D texture = new Texture2D(image);
renderer.readFrameBuffer(offBuffer, image.getData(0));
//image.setUpdateNeeded();
try {
savePng(path, image);
} catch (IOException ex) {
log.error("Error generating icon!", ex);
}
path = null;
}
}
public void generateIcon(Spatial model, File path) {
// Vector3f bound = ((BoundingBox) model.getWorldBound()).getExtent(new Vector3f()).multLocal(2);
// float max = Math.max(Math.max(bound.x, bound.y), bound.z);
// float resize = baseSize / max;
// model.scale(resize);
// setup framebuffer's scene
root.detachAllChildren();
root.attachChild(model);
this.path = path;
this.time = 0;
}
protected void updateCamera() {
BoundingBox bb = (BoundingBox) root.getWorldBound();
Vector3f min = bb.getMin(null);
Vector3f max = bb.getMax(null);
float xSize = Math.max(Math.abs(min.x), Math.abs(max.x));
float ySize = max.y - min.y;
float zSize = Math.max(Math.abs(min.z), Math.abs(max.z));
float size = ySize * 0.5f;
size = Math.max(size, xSize);
size = Math.max(size, zSize);
// In the projection matrix, [1][1] should be:
// (2 * Zn) / camHeight
// where Zn is distance to near plane.
float m11 = offView.getCamera().getViewProjectionMatrix().m11;
// We want our position to be such that
// 'size' is otherwise = cameraHeight when rendered.
float z = m11 * size;
// Add the z extents so that we adjust for the near plane
// of the bounding box... well we will be rotating so
// let's just be sure and take the max of x and z
//float offset = Math.max(bb.getXExtent(), bb.getZExtent());
//z += offset;
// This creates problems because it makes way too much
// space around the tree. A proper solution would require
// a bunch of math and in the end would also have to be duplicated
// on the quad generation side or somehow stored with the atlas.
Vector3f center = bb.getCenter();
float sizeOffset = 0;
Vector3f camLoc = new Vector3f(0, center.y + sizeOffset, z);
offView.getCamera().setLocation(camLoc);
if (debugBounds) {
WireBox box;
if (wireBounds == null) {
box = new WireBox();
wireBounds = new Geometry("wire box", box);
Material mat = GuiGlobals.getInstance().createMaterial(ColorRGBA.Yellow, false).getMaterial();
wireBounds.setMaterial(mat);
root.attachChild(wireBounds);
} else {
box = (WireBox) wireBounds.getMesh();
}
box.updatePositions(bb.getXExtent(), bb.getYExtent(), bb.getZExtent());
box.setBound(new BoundingBox(new Vector3f(0, 0, 0), 0, 0, 0));
wireBounds.setLocalTranslation(bb.getCenter());
//wireBounds.setLocalRotation(leafGeom.getLocalRotation());
}
}
protected void savePng(File f, Image img) throws IOException {
try (OutputStream out = new FileOutputStream(f)) {
JmeSystem.writeImageFile(out, "png", img.getData(0), img.getWidth(), img.getHeight());
log.debug("Icon saved at {}", f.getPath());
}
}
protected Image createFrameBufferImage(FrameBuffer fb) {
int width = fb.getWidth();
int height = fb.getHeight();
int size = width * height * 4;
ByteBuffer buffer = BufferUtils.createByteBuffer(size);
Image.Format format = fb.getColorBuffer().getFormat();
// I guess readFrameBuffer always writes in the same
// format regardless of the frame buffer format
format = Image.Format.BGRA8;
return new Image(format, width, height, buffer);
}
}Seems related to issues discussed in:
So, if those are separate geometry then make sure the trunk is in the opaque bucket and tree in the transparent bucket… else you may need to turn on the alpha threshold.
It’s a single geometry. I enabled alpha threshold and it fixed the issue.
Thank you so much.
I seem to have yet another problem 
In this specific model, this is how my model looks in real-time and when exported to jpg.
but when exporting to png it looks like:
This happens only in this specific model.
I am clueless what may be the issue, anybody have any guess what may cause this ?
Edit:
I am using the same code I provided above.
It’s acting like you are exporting RGB as BGR or vice-versa… ie: red and blue are flipped.
hmm, then shouldn’t same effect happen when exporting to jpg too.
As you know this is how they are exported : jpg uses BufferedImage.TYPE_INT_BGR and png uses BufferedImage.TYPE_4BYTE_ABGR
BufferedImage awtImage;
if (format.equals("jpg")) {
JPEGImageWriteParam jpegParam = (JPEGImageWriteParam) writeParam;
jpegParam.setCompressionMode(ImageWriteParam.MODE_EXPLICIT);
jpegParam.setCompressionQuality(0.95f);
awtImage = new BufferedImage(width, height, BufferedImage.TYPE_INT_BGR);
Screenshots.convertScreenShot2(imageData.asIntBuffer(), awtImage);
awtImage = verticalFlip(awtImage);
} else {
awtImage = new BufferedImage(width, height, BufferedImage.TYPE_4BYTE_ABGR);
Screenshots.convertScreenShot(imageData, awtImage);
}
Edit:
I wonder if Screenshots.convertScreenShot might be faulty:
Was the coloring right before your changes?
Just tested it and yes it is right.
This is the png file exported with JME 3.2 (off-course there no transparency) :
I am putting the old code here:
@Override
public void writeImageFile(OutputStream outStream, String format, ByteBuffer imageData, int width, int height) throws IOException {
BufferedImage awtImage = new BufferedImage(width, height, BufferedImage.TYPE_INT_BGR);
Screenshots.convertScreenShot2(imageData.asIntBuffer(), awtImage);
ImageWriter writer = ImageIO.getImageWritersByFormatName(format).next();
ImageWriteParam writeParam = writer.getDefaultWriteParam();
if (format.equals("jpg")) {
JPEGImageWriteParam jpegParam = (JPEGImageWriteParam) writeParam;
jpegParam.setCompressionMode(ImageWriteParam.MODE_EXPLICIT);
jpegParam.setCompressionQuality(0.95f);
}
awtImage = verticalFlip(awtImage);
ImageOutputStream imgOut = new MemoryCacheImageOutputStream(outStream);
writer.setOutput(imgOut);
IIOImage outputImage = new IIOImage(awtImage, null, null);
try {
writer.write(null, outputImage, writeParam);
} finally {
imgOut.close();
writer.dispose();
}
}
Edit:
My doubt is even Screenshots.convertScreenShot() is not working as it is supposed to or Java’s ImageIO may have issue ?
TYPE_4BYTE_ABGR
Represents an image with 8-bit RGBA color components with the colors Blue, Green, and Red stored in 3 bytes and 1 byte of alpha.
I didn’t read the whole post, but is this what we want?
Shouldn’t we use
intTYPE_INT_ARGB
Represents an image with 8-bit RGBA color components packed into integer pixels.
And
``
TYPE_INT_RGB
Represents an image with 8-bit RGB color components packed into integer pixels.
for non alpha.
Or is there any reason for which BGR is preferable?
In your newer code, I wonder what happens if you make it:
Screenshots.convertScreenShot(imageData.asIntBuffer(), awtImage);
…I don’t know if that nukes the alpha or not.
Or we may go back and look at the 3.0 code when it worked for PNG and see what’s different.
Okay guys, I am giving up on JME’s method to write image files.
I found this ImageUtil class created by @NemesisMate and tested it on bunch of models and it works like a charm.
This is the example model exported to PNG with ImageUtil.
I mean, it used to work… that I’m sure of.