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Convert C++ IplImage.ImageData to jbyteArray

asked 2013-09-18 13:06:46 -0500

Exspecto gravatar image

updated 2013-09-18 13:09:13 -0500


I using a c++ dll provided by a customer that connects to a USB-video camera. The image captured by the camera uses OpenCV contructs such as IplImage. Since I need to create a Java applet to display the image provide by this .dll, I've create a JNI file that implements equivalent methods to the C++ code (see below). The problem with my C++ implementation is that I can't convert IplImage->imageData to a jbyteArray (it's always "null"). I'm not sure if I'm going about it the right way. I've used the JavaCV project but it's an overkill for my simple Java Applet. I would prefer to convert IplImage->imageData to some native Java structure instead. I can't figure out what is wrong with my C++ implementation. Suggestions?

MY JAVA MapleJNI Class:

public class MapleJNI {



 public static native byte maple_connect(); 
 public static native void maple_disconnect(); 
 public static native byte maple_set_capture_mode(byte mode); 
 public static native byte maple_capture_image(byte image_type, byte[] img_buf); 
 public static native void maple_crop_image(byte[] img_buf); 
 [ ... ]

C++ - Code ".cpp" File (JNI Implementation)

JNIEXPORT jbyte JNICALL Java_maple_MapleJNI_maple_1capture_1image (JNIEnv *env, jclass, jbyte mode, jbyteArray image){

    UCHAR result = maple_capture_image((UCHAR) mode, (UCHAR *) rgb_img->imageData);
            // NOTE:  rgb_img is a global variable (for now) and I've proven that is
            //        does contain the captured image from the video camera

    jsize len = 0;

    if (result == STATUS_NO_ERROR)
        // Crop top/bot rows of image
        // (set to black) to avoid jitter
        maple_crop_image((UCHAR *) rgb_img->imageData);

        len = sizeof(rgb_img->imageData);

        image = env->NewByteArray(len);
                    // Copy "rgb_image" to java native jbyteArray "image"
        env->SetByteArrayRegion(image, 0, len, (jbyte*) rgb_img->imageData);

    return result;


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answered 2013-09-18 13:13:29 -0500

berak gravatar image

i think, your len is wrong:

len = sizeof(rgb_img->imageData);

since rgb_img->imageData is a pointer to uchar, sizeof(rgb_img->imageData) will give 4 on a 32 bit system, and 8 on 64 bit one. definitely not, what you wanted, right ?

the desired len is probably:

width * height * nChannels


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Thanks for pointing that out. Would "rgb_img->imageSize" be the better option? Documentation states :

imageSize: Image data size in bytes. For interleaved data, this equals image->height * image->widthStep


Exspecto gravatar imageExspecto ( 2013-09-18 13:49:39 -0500 )edit

sounds good, too !

berak gravatar imageberak ( 2013-09-18 13:54:56 -0500 )edit

The fix suggested resolves the initial problem. Now I wonder if the converted IplImage->imageData to a jbyteArray is something that Java code can correctly convert into an image. I performed the following on the converted jbyteArray to display the image into an applet:

//NOTE buffImg is of class BufferedImage

// Get byte byte[] image = null; usb.maple_capture_image(ImageType.RGB.getValue(), image);

if (image != null){ // Received byte array from camera. Let's display in applet. We need to convert byte array ByteArrayInputStream in = new ByteArrayInputStream(image); buffImg =;

if (buffImg != null) {
 graph = buffImg.createGraphics();
    System.out.println("Buffer is NULL");


Exspecto gravatar imageExspecto ( 2013-09-18 14:15:50 -0500 )edit
BufferedImage buffImg = new BufferedImage(width,height, BufferedImage.TYPE_3BYTE_BGR); // type right ?
image.getRaster().setDataElements(0, 0, width,height, image );
berak gravatar imageberak ( 2013-09-18 14:21:09 -0500 )edit


This worked like a charm!

Thank your for your help!

Exspecto gravatar imageExspecto ( 2013-09-18 14:36:20 -0500 )edit

pleasure !

berak gravatar imageberak ( 2013-09-18 14:38:29 -0500 )edit
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Asked: 2013-09-18 13:06:46 -0500

Seen: 1,431 times

Last updated: Sep 18 '13