Unable to render square in directx9 - directx

Question

// this is the function used to render a single frame
void render_frame(void)
{
init_graphics();
d3ddev->Clear(0, NULL, D3DCLEAR_TARGET, D3DCOLOR_XRGB(255, 0, 0), 1.0f, 0);
d3ddev->BeginScene();
// select which vertex format we are using
d3ddev->SetFVF(CUSTOMFVF);
// select the vertex buffer to display
d3ddev->SetStreamSource(0, v_buffer, 0, sizeof(CUSTOMVERTEX));
// copy the vertex buffer to the back buffer
d3ddev->DrawPrimitive(D3DPT_TRIANGLESTRIP, 0, 2);
// d3ddev->DrawPrimitive(D3DPT_TRIANGLESTRIP, 0, 1);
d3ddev->EndScene();
d3ddev->Present(NULL, NULL, NULL, NULL);
}
// this is the function that puts the 3D models into video RAM
void init_graphics(void)
{
// create the vertices using the CUSTOMVERTEX struct
CUSTOMVERTEX vertices[] =
{
{ 100.f, 0.f, 0.f, D3DCOLOR_XRGB(0, 0, 255), },
{ 300.f, 0.f, 0.f, D3DCOLOR_XRGB(0, 0, 255), },
{ 300.f, 80.f, 0.f, D3DCOLOR_XRGB(0, 0, 255), },
{ 100.f, 80.f, 0.f, D3DCOLOR_XRGB(0, 0, 255), },
};
// create a vertex buffer interface called v_buffer
d3ddev->CreateVertexBuffer(6 * sizeof(CUSTOMVERTEX),
0,
CUSTOMFVF,
D3DPOOL_MANAGED,
&v_buffer,
NULL);
VOID* pVoid; // a void pointer
// lock v_buffer and load the vertices into it
v_buffer->Lock(0, 0, (void**)&pVoid, 0);
memcpy(pVoid, vertices, sizeof(vertices));
v_buffer->Unlock();
}
Can't render a square for some reason. I've searched for an hour but can't find the answer.
https://i.imgur.com/KCKZSrJ.jpg
Does anybody know how to render it? I'm using directx9.
Tried using DrawIndexPrimitive but It has the same result.

Answer 1

There's likely a few things going on here:
You do not set a Vertex or Pixel Shader, so you are using the legacy fixed-function render pipeline. This pipeline requires you set the view/projection matrices with SetTransform. Since you haven't done that, the vertex positions you provide in 'screens space' don't mean what you think they mean. See The Direct3D Transformation Pipeline.
You are not setting the backface culling mode via SetRenderState so it's defaulting to D3DCULL_CCW (i.e. cull counter-clockwise winding triangles). As such, your vertex positions are resulting in one of the triangles being rejected. You may want to to call SetRenderState(D3DRS_CULLMODE, D3DCULL_NONE); getting started.
You are using TRIANGLESTRIP and only 4 points. You may find it easier to get correct initially by using TRIANGELIST and 6 points.