J2ME Screenshot of a Canvas

[mtgbe]

Hi,

I'm currently trying to

• take a screenshot of a J2ME Canvas screen
• converting it as a PNG (using PNGEncoder from http://www.chrfr.de/software/midp_png.html)
• then storing it in the RMS.

My objective : I want that the user make a signature on touchscreen (using stylet), and then send it to a server.

I saw a method getScreenShot(), that seems to do what i first want, but can't figure how it works (about 2line...) and how to use it...

    public Image getScreenShot() {
        Image screenshot = Image.createImage(getWidth(), getHeight());
        Graphics g = screenshot.getGraphics();
        paint(g);
        return Image.createImage(screenshot);
    }

Has anyone did this ?

Thanks in advance

[rahulvala]

I think there is no direct way of taking screenshot of canvas in J2ME..

You can see the below two discussions..

http://discussion.forum.nokia.com/fo...2me+screenshot

http://discussion.forum.nokia.com/fo...2me+screenshot

Best regards,

[mtgbe]

Thanks for your response

I've already read those.

The guy says :

You can however take a "screenshot" of the Canvas, if you're using one: http://discussion.forum.nokia.com/fo...ght=screenshot

But i've never achieved this...

I really think that this is possible, but don't have time to make more inquiry, and hope someone has already done this...

[grahamhughes]

I saw a method getScreenShot(), that seems to do what i first want, but can't figure how it works (about 2line...) and how to use it...

    public Image getScreenShot() {
        Image screenshot = Image.createImage(getWidth(), getHeight());
        Graphics g = screenshot.getGraphics();
        paint(g);
        return Image.createImage(screenshot);
    }

Assuming your Canvas's paint() method works correctly, this should return an Image identical to the screen contents. Are you saying it doesn't? It returns an Image object, not a PNG encoded image.

You'll need to use Image.getRGB() to get the pixel data in an int[]. That int[] then needs to be split into four byte[]s to make it compatible with the PNG encoder. However, you could simply store the int[] (along with the width and height), and not bother encoding as a PNG. You don't appear to need the data in PNG format.

Graham.

[mtgbe]

I thought about PNG for compression. Because when this will be sent to the server, what kind of image will i get there ? BMP ?

[grahamhughes]

Ah yes, if you're sending to a server, you will need to encode it as some format. Note, however, that the PNG encoder code you have does not compress the image. It only encodes it. It will not make the data smaller.

You can create the four byte arrays like:

// get pixel data
int width = img.getWidth();
int height = img.getHeight();
int[] pixels = new int[width * height];
img.getRGB(pixels, 0, width, 0, 0, width, height);

// separate channels
byte[] red   = new byte[pixels.length];
byte[] green = new byte[pixels.length];
byte[] blue  = new byte[pixels.length];
byte[] alpha = new byte[pixels.length];

for (int i = 0; i < pixels.length; i++) {
    int argb = pixels[i];
    alpha[i] = (byte) (argb >> 24);
    red[i]   = (byte) (argb >> 16);
    green[i] = (byte) (argb >>  8);
    blue[i]  = (byte) (argb);
}

Graham.

[mtgbe]

Ok, thanks a lot, i'll give it a try next week and will post my experience.

If my screen is 320x240, that will make a 76.800 int array. Won't it be too much for a as small device ?

Integer is 16 bits no ? I see that the shifts reveals 32bits... ?

Have you heard about a good image compressor (and also not too hungry in ressources) ?

[grahamhughes]

If my screen is 320x240, that will make a 76.800 int array. Won't it be too much for a as small device ?

Yes, it might be.

Integer is 16 bits no ? I see that the shifts reveals 32bits... ?

A Java int is 32 bits. So, the int[] will consume about 300k. The four byte arrays will need the same again. The PNG image data will also be about 300k. Make sure to add:

pixels = null;

after creating the byte arrays, and before encoding the PNG, or you'll need 900k of memory. Even then, the peak memory usage will be around 600k.

Uploading it to a server by HTTP will also consume 300k, in order to buffer the POST data.

Have you heard about a good image compressor (and also not too hungry in ressources) ?

No, I haven't seen a complete image encoder that includes compression. You'd need to write one.

Assuming the image data you want to store is just two-colour (a signature drawn on a touch screen), then this PNG encoder is very inefficient. It uses 32 bits per pixel, but you need only one bit. You could fit the PNG in around 10k if you encode it differently (without compression). However, you'd need to re-write the PNG encoder.

Graham.

[mtgbe]

allright, i see.

Thanks a lot for all ur explanations

I'll give it a try and post my feedback

[mtgbe]

Ok, I've found some interresting code to generate a monochrome bitmap inside zxing (barcode reader).

I'll keep in touch.