C++ – How to use “billboards” to create a spherical object on the screen

cdirectx

I am tasked with making a sun/moon object flow across the screen throughout a time-span (as it would in a regular day). One of the options available to me is to use a "billboard", which is a quad that is always facing the camera.

I have yet to use many direct x libraries or techniques. This is my first graphics project. How does this make sense? And how can you use this to move a sun object across a screen?

Thanks 🙂 This will be run on windows machines only and I only have the option for direct x (9).

I have gotten this half working. I have a sun image displaying, but it sits at the front of my screen overtop of 80% of my screen, no matter which way my camera is pointing. I'm looking down towards the ground? Still a huge sun there. Why is this? Here is the code I used to create it…

void Sun::DrawSun()
{
    std::wstring hardcoded = L"..\\Data\\sun.png";
    m_SunTexture = MyTextureManager::GetInstance()->GetTextureData(hardcoded.c_str()).m_Texture;


LPD3DXSPRITE sprite = NULL;

if (SUCCEEDED(D3DXCreateSprite(MyRenderer::GetInstance()->GetDevice(), &sprite)))
{
    //created!
}

sprite->Begin(D3DXSPRITE_ALPHABLEND);

D3DXVECTOR3 pos;

pos.x = 40.0f;
pos.y = 20.0f;
pos.z = 20.0f;

HRESULT someHr;
someHr = sprite->Draw(m_SunTexture, NULL, NULL, &pos, 0xFFFFFFFF);
sprite->End();
}

Obviously, my position vector is hardcoded. Is this what I need to be changing? I have noticed in the documentation the possibility of D3DXSPRITE_BILLBOARD rather than D3DXSPRITE_ALPHABLEND, will this work? Is it possible to use both?

As per the tutorial mentioned in an earlier post, D3DXSPRITE is a 2d object, and probably will not work for displaying within the 3d world? What is a smarter alternative?

Best Answer

The easiest way to do a screen aligned quad is by using Point Sprites with texture rewrite.
I never did that with DirectX but in OpenGL, enabling point sprites is a matter of 2 API calls that look like this. 

glEnable(GL_POINT_SPRITE);
glTexEnvi(GL_POINT_SPRITE, GL_COORD_REPLACE, true);

With this mode enabled, You draw a single vertex and instead of a point, a screen aligned quad is renderd. The coordinate replace thing means that the rendered quad is processed with texture coordinates. This means that you can place any texture on the quad. Usually you'll want something with an alpha channel to blend into the background seamlessly.

There should be an equivalently easy way to do it in D3D. In addition, if you write a shader, it may allow you do do some additional stuff like discard some of the pixels of the texture.

This tutorial might help
Also, google.

--Edit
To transform a quad to any other shape, use the alpha channel of the texture. If the alpha is 0, the pixel is not visible. You can't add an alpha channel to a JPEG but you can do it to a PNG. Here's an example:

sun with alpha channel http://www.shiny.co.il/shooshx/sun.png

If you'll open this image in photoshop you'll see that the background is invisible.
Using alpha blending might cause some problems if you have other things going on in the scene so if that happens, you can write a simple fragment shader which discards the pixels with alpha==0.

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