Creating a custom mouse cursor with LWJGL2 in Java

Question

I have been trying to create a custom mouse cursor in my LWJGL2 application running under Linux and I am almost there. I have implemented the following method that I call right after creating the game window:

public void loadCursor(BufferedImage img) throws LWJGLException
{
    final int w = img.getWidth();
    final int h = img.getHeight();

    int rgbData[] = new int[w * h];

    for (int i = 0; i < rgbData.length; i++)
    {
        int x = i % w;
        int y = h - 1 - i / w; // this will also flip the image vertically

        rgbData[i] = img.getRGB(x, y);
    }

    IntBuffer buffer = BufferUtils.createIntBuffer(w * h);
    buffer.put(rgbData);
    buffer.rewind();

    Cursor cursor = new Cursor(w, h, 2, h - 2, 1, buffer, null);

    Mouse.setNativeCursor(cursor);
}

The resulting cursor is almost perfect, except for a horizontal blank line in the middle.

Note: The image is being flipped vertically in my code because otherwise the cursor is displayed upside down in OpenGL. I have seen other implementations that flip the IntBuffer instead of doing this in the for-loop but they lead to the same result for me.

I don't think it has to do with screen tearing either since I enabled VSync and the blank line is always at 50% of the cursor's height (there is no flickering).

Can anybody tell what I am doing wrong or if my code example is missing something essential in order to find the problem? Thank you!

Answer 1

It's not that your code might be wrong but the size of your buffered image. LWJGL imaging uses increments of 2 (2, 4, 8, 16, 32, 64, 128...), so then your image should be just that. I have used your code and it works just fine.

Answer 2

A guess as to why that line is there is that your flipping algorithm doesn't handle odd/middle lines well. I recommend the classical algorithm for flipping, for(int i = 0;i<length/2;i++) array[i] = array[array.length-i - 1] (psuedocode).

Answer 3

At first glance, this looks far too complicated. Lwjgl should have something to the effect of 'getmouseposition()'. Call this function every iteration of your game loop, cast or set it as a Vec2d, then use that position to blit or draw the mouse you want. All of your blits should be called every game iteration in some kind of draw loop.

Mind you, I was trying to use Lwjgl in java, and ended up needing opengl and a shader program to discard the background color of the images I was blitting.

As I recall, with opengl there were issues with blitting images of certain sizes - I think the dimensions had to increase by set numbers for it to blit correctly. Since, if I remeber correctly, Lwjgl uses opengl, this could be what your problem is.