Comprehensive Guide to Using Cubemaps with DirectX 9 and C++: 4x3 Cross Format, Reflections, and Sky Spheres

A cubemap is a spherical image representation method used in 3D graphics programming, especially to implement effects such as reflection, skybox/sky sphere, and environment mapping. In this article, we will provide a comprehensive answer to the questions of what a cubemap is, how to create it, which formats are supported, and how to apply it using DirectX 9 and C++.

What is a Cubemap?

A cubemap is a special texture created by combining 6 different images (like 6 cameras oriented towards the faces of a cube) taken from the center of a sphere outwards. These images represent:

+X (right)
-X (left)
+Y (up)
-Y (down)
+Z (forward)
-Z (back)

directions.

These textures are used together to create environmental reflection or sky effects on 3D objects.

️ What is 4x3 Cross Format?

Cubemaps are often stored in special arrangements:

The 4x3 Cross Format can be visualized as follows:

     +Y
 -X +Z +X -Z
     -Y

This format is especially used to create cubemaps from offline rendered sky images. It contains 6 faces in a single image. DirectX can automatically separate the faces to use this format, or they must be manually cut with software.

⚙️ How to Create a Cubemap with DirectX 9?

Cubemap Texture Definition:

LPDIRECT3DCUBETEXTURE9 pCubeTexture = NULL;
d3ddev->CreateCubeTexture(
    256, 0, D3DUSAGE_RENDERTARGET,
    D3DFMT_A8R8G8B8, D3DPOOL_DEFAULT, &pCubeTexture, NULL);

Loading Data to Faces:

D3DXLoadSurfaceFromFile(
    pCubeTexture->GetCubeMapSurface(D3DCUBEMAP_FACE_POSITIVE_X, 0),
    NULL, NULL, "posx.jpg", NULL, D3DX_DEFAULT, 0, NULL);

You must do the same for all 6 faces.

Usage in Shader (VS/PS 2.0)

samplerCUBE CubeMapSampler = sampler_state
{
    Texture = ;
};

float4 PS_Main(float3 reflectDir : TEXCOORD0) : COLOR0
{
    return texCUBE(CubeMapSampler, reflectDir);
}

Calculating Cube Mapping Direction (C++) When calculating the reflection direction:

float3 view = normalize(eyePos - worldPos);
float3 reflectDir = reflect(view, normal);

Cubemap Usage Areas

Usage Area	Description
Skybox	Sky and distant landscapes in the game
Reflection Map	Environmental reflection on surfaces such as metal and glass
Refraction Map	Refraction effect in semi-transparent objects such as water
IBL (Image Based Lighting)	Environmental lighting in PBR systems

Cubemap Conversion and Tools

Photoshop + NVIDIA DDS Plugin
CMFT Studio (https://github.com/dariomanesku/cmftStudio)
DirectX Texture Tool (DxTex.exe): You can manually load the faces and create .dds

Cubemap Render-to-Texture (RTT) for Dynamic Reflection

Render your scene 6 times with 90° FOV in different directions
Render to each face with SetRenderTarget()
Create real-time reflection using this dynamic cubemap with Shader

This method is especially used to reflect the surroundings of moving objects.

✅ Conclusion

Cubemap technology is a very powerful technique for creating impressive environmental visuals in the DirectX 9 environment. It can be used in many areas such as reflection, sky, and lighting. It can be used with formats such as 4x3 Cross or DDS based textures. The above steps provide a basic starting point for loading a cubemap into your D3D device and using it in a shader when developing an application with C++.