Android SetPixels() Explanation and Example?

Question

I am trying to set a region of pixels in a mutable bitmap a different color in my android app. Unfortunately I cannot get setPixels() to work properly. I am constantly getting ArrayOutOfBoundsExceptions. I think it may have something to do with the stride, but I'm really not sure. That is the only parameter that I still don't understand. The only other post I have seen on setPixels (not setPixel) is here: drawBitmap() and setPixels(): what's the stride? and it did not help me. I tried setting the stride as 0, as the width of the bitmap, as the width of the bitmap - the area i'm trying to draw and it still crashes. Here is my code:

public void updateBitmap(byte[] buf, int offset, int x, int y, int width, int height) {
    // transform byte[] to int[]
    IntBuffer intBuf = ByteBuffer.wrap(buf).asIntBuffer();
    int[] intarray = new int[intBuf.remaining()];
    intBuf.get(intarray);                                       

    int stride = ??????
    screenBitmap.setPixels(intarray, offset, stride, x, y, width, height); // crash here

My bitmap is mutable, so I know that is not the problem. I am also certain that my byte array is being properly converted to an integer array. But I keep getting ArrayOutOfBoundsExceptions and I don't understand why. Please help me figure this out

EDIT - here is how I construct the fake input:

int width = 1300;
int height = 700;
byte[] buf = new byte[width * height * 4 * 4]; // adding another * 4 here seems to work... why?     
for (int i = 0; i < width * height * 4 * 4; i+=4) {
    buf[i] = (byte)255;
    buf[i + 1] = 3;
    buf[i + 2] = (byte)255;
    buf[i + 3] = 3;         
}
//(byte[] buf, int offset, int x, int y, int width, int height)  - for reference        
siv.updateBitmap(buf, 0, 0, 0, width, height);

So the width and height are the correct amount of ints (at least it should be).

EDIT2 - here is the code for the original creation of screenBitmap:

public Bitmap createABitmap() {
int w = 1366;
int h = 766;

byte[] buf = new byte[h * w * 4];

for (int i = 0; i < h * w * 4;i+=4) {
        buf[i] = (byte)255;
    buf[i+1] = (byte)255;
    buf[i+2] = 0;
    buf[i+3] = 0;
}

DisplayMetrics metrics = new DisplayMetrics();
getWindowManager().getDefaultDisplay().getMetrics(metrics);

IntBuffer intBuf = ByteBuffer.wrap(buf).asIntBuffer();  
int[] intarray = new int[intBuf.remaining()];
intBuf.get(intarray);                                           

Bitmap bmp = Bitmap.createBitmap(metrics, w, h, itmap.Config.valueOf("ARGB_8888"));
bmp.setPixels(intarray, 0, w, 0, 0, w, h);
return bmp;
}

It seems to work in this instance, not sure what the difference is

Stride should be width. It's the amount in the 1d array you need to go forward to be down one, when it pretends to be a 2d array. — Tatarize, Oct 12 '15 at 01:48
That doesn't work because setPixels() takes an intarray. Period. Your 4 color sample byte array is turning into various shades of blue. 0baaaaaaaarrrrrrrrggggggggbbbbbbbbb — Tatarize, Oct 12 '15 at 01:53

User · Answer 1 · 2020-02-27T11:16:27.413

Short I think, stride is the width of the image contained in pixels.

long

What I take from the documentation, the algorithm should work somehow like this:

public void setPixels (int[] pixels, 
            int offset, 
            int stride, 
            int x, 
            int y, 
            int width, 
            int height)
    for (int rowIndex = 0; rowIndex < height; rowIndex++) {
        int rowStart = offset + stride * rowIndex; // row position in pixels
        int bitmapY = y + rowIndex; // y position in the bitmap
        for (int columnIndex = 0; columnIndex < width; columnIndex++) {
            int bitmapX = x + columnIndex; // x position in the bitmap
            int pixelsIndex = rowStart + columnIndex; // position in pixels
            setPixel(bitmapX, bitmapY, pixels[pixelsIndex]);

        }
    }
}

At least this is what I would do with these arguments because is allows you to have one pixels as source and cut out different images in different sizes. Feel free to correct the algorithm if I am wrong.

So, say you have an image with pixelsWidth and pixelsHeight in pixels. Then you want to copy a section at pixelsX and pixelsY with width and height to a bitmap at position bitmapX and bitmapY. This should be the call:

bitmap.setPixels(
    pixelsWidth * pixelsY + pixelsX, // offset
    pixelsWidth, // stride
    bitmapX, // x
    bitmapY, // y
    width,
    height)

yushulx · Answer 2 · 2013-09-26T05:47:11.530

1

probably it should be:

screenBitmap.setPixels(intarray, 0, width / 4, x, y, width / 4, height);

because you have converted byte to int. your error is ArrayOutOfBoundsExceptions. check whether the size intBuf.remaining() = width * height / 4.

edited Sep 26 '13 at 05:47

answered Sep 26 '13 at 05:40

yushulx

11,695
8
37
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My array size is correct I think. If I create a fake input of a 1300 x 700 byte input (so 1300 x 700 x 4 bytes = 3,640,000), that creates an intarray of length 91,000, which is one fourth the total. In ARGB_8888 format each pixel is one 32 bit int, so 91,000 = 1300 x 700, so that should be enough pixel data to fill a 1300x700 section of the bitmap. However when I do 1300 x 700 x 4 x 4 pixels it fills up the right amount of space without crashing. Is my math wrong somehow? -EDIT: see above for how I construct the fake input – Trevor Sep 26 '13 at 05:56
which line your code crashed at?intBuf.remaining() or screenBitmap.setPixels? and what is your stride value? it cannot be displayed – yushulx Sep 26 '13 at 06:25
do the width and height values equal to the size of screenBitmap? – yushulx Sep 26 '13 at 06:29
my code crashes on bitmap.setPixels(...). I have tried numerous things with stride and none of them seem to work for the data I am receiving over the socket. For some reason when I add 4x as many pixels in the fake data it works. I do not understand why I need to do height * width * 16 to write that new section of the bitmap to my image. Currently I am receiving a 20x20 pixel block of data over the socket. This comes out to 1600 bytes (20 x 20 pixels x 4 bytes per pixel), which I have verified to be 400 ints. So numerically it seems like it should work, but its not... – Trevor Sep 26 '13 at 06:32
what are the width and height of your screenBitmap? How did you create it? – yushulx Sep 26 '13 at 06:33
See edit above for how I created screen bitmap. I posted the code – Trevor Sep 26 '13 at 06:35
could you have a try to make all (width, height) to (640, 480) including bitmap creation and fake construction? – yushulx Sep 26 '13 at 06:43
let us [continue this discussion in chat](http://chat.stackoverflow.com/rooms/38077/discussion-between-trevor-and-yushulx) – Trevor Sep 26 '13 at 06:45

score 0 · Answer 3 · answered Sep 26 '13 at 05:33

0

If you're trying to draw on a bitmap, your best approach would be to abandon this method and use a canvas instead:

Canvas canvas = new Canvas(screenBitmap);

You can then draw specific points (if you want to draw a pixel), or other shapes like rectangles, circles, etc:

canvas.drawPoint(x, y, paint);

etc

Hope this helps.

answered Sep 26 '13 at 05:33

Gil Moshayof

16,633
4
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Well I'm not trying to draw on the bitmap. I'm trying to simulate a real time video feed, so I will be updating the bitmap with new image data as it comes in. So I don't want to draw, I want to set the pixels to their new value – Trevor Sep 26 '13 at 05:37

Android SetPixels() Explanation and Example?

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