Simple rendering

Here we learn how to render a triangle.

Setup

First create a new project using the instructions from the first two lessons: The basic game loop and Adding the DirectX Tool Kit which we will use for this lesson.

Background

In order to do a draw operation with Direct3D 11, we need to provide the following objects and settings:

A vertex buffer containing the vertices of the elements to draw.
The input layout that describes the memory layout of the vertices in the vertex buffer.
A primitive topology setting that indicates how to interpret the individual vertices (as a point, a line, a triangle, etc.)
A compiled vertex shader program
A compiled pixel shader program
Set any required state objects for rasterizer state, depth/stencil state, blend state, and sampler state (if using textures)

For this lesson, the BasicEffect object will provide the vertex and pixel shader programs, VertexPositionColor will provide the input layout, CommonStates will provide the 'stock' state objects, and PrimitiveBatch will provide the vertex buffer and primitive topology.

Note that since the input layout is the bridge between the vertex buffer data and the vertex shader program, this Direct3D object is created with information about both.

Drawing a triangle

In the Game.h file, add the following variables to the bottom of the Game class's private declarations:

using VertexType = DirectX::VertexPositionColor;

std::unique_ptr<DirectX::CommonStates> m_states;
std::unique_ptr<DirectX::BasicEffect> m_effect;
std::unique_ptr<DirectX::PrimitiveBatch<VertexType>> m_batch;
Microsoft::WRL::ComPtr<ID3D11InputLayout> m_inputLayout;

In Game.cpp, add to the TODO of CreateDevice:

m_states = std::make_unique<CommonStates>(m_d3dDevice.Get());

m_effect = std::make_unique<BasicEffect>(m_d3dDevice.Get());
m_effect->SetVertexColorEnabled(true);

void const* shaderByteCode;
size_t byteCodeLength;

m_effect->GetVertexShaderBytecode(&shaderByteCode, &byteCodeLength);

DX::ThrowIfFailed(
        m_d3dDevice->CreateInputLayout(VertexType::InputElements,
            VertexType::InputElementCount,
            shaderByteCode, byteCodeLength,
            m_inputLayout.ReleaseAndGetAddressOf()));

m_batch = std::make_unique<PrimitiveBatch<VertexType>>(m_d3dContext.Get());

In Game.cpp, add to the TODO of OnDeviceLost:

m_states.reset();
m_effect.reset();
m_batch.reset();
m_inputLayout.Reset();

In Game.cpp, add to the TODO of Render:

m_d3dContext->OMSetBlendState( m_states->Opaque(), nullptr, 0xFFFFFFFF );
m_d3dContext->OMSetDepthStencilState( m_states->DepthNone(), 0 );
m_d3dContext->RSSetState( m_states->CullNone() );

m_effect->Apply(m_d3dContext.Get());

m_d3dContext->IASetInputLayout(m_inputLayout.Get());

m_batch->Begin();

VertexPositionColor v1(Vector3(0.f, 0.5f, 0.5f), Colors::Yellow);
VertexPositionColor v2(Vector3(0.5f, -0.5f, 0.5f), Colors::Yellow);
VertexPositionColor v3(Vector3(-0.5f, -0.5f, 0.5f), Colors::Yellow);

m_batch->DrawTriangle(v1, v2, v3);

m_batch->End();

Build and run to see a simple yellow triangle rendered in 2D.

Screenshot of triangle

You don't have to use a type alias here like VertexType and you can just use DirectX::VertexPositionColor for the header and VertexPositionColor in the cpp file directly. I use the alias to simplify the tutorial a bit later on.

Pixel vs. normalized coordinates

The image above is drawn using coordinates that are independent of the screen resolution and range from -1 to +1. Resizing the window will result in the same image scaled to the new window. If instead you want to draw using screen pixel coordinates (which match the coordinate system used by SpriteBatch), then:

In Game.cpp, add to the TODO of CreateResources:

Matrix proj = Matrix::CreateScale( 2.f/float(backBufferWidth),
   -2.f/float(backBufferHeight), 1.f)
   * Matrix::CreateTranslation( -1.f, 1.f, 0.f );
m_effect->SetProjection(proj);

The projection matrix can also be created with Matrix::CreateOrthographicOffCenter(0.f, float(backBufferWidth), float(backBufferHeight), 0.f, 0.f, 1.f);

In Game.cpp, modify the TODO of Render:

m_d3dContext->OMSetBlendState( m_states->Opaque(), nullptr, 0xFFFFFFFF );
m_d3dContext->OMSetDepthStencilState( m_states->DepthNone(), 0 );
m_d3dContext->RSSetState( m_states->CullNone() );

m_effect->Apply(m_d3dContext.Get());

m_d3dContext->IASetInputLayout(m_inputLayout.Get());

m_batch->Begin();

VertexPositionColor v1(Vector3(400.f, 150.f, 0.f), Colors::Yellow);
VertexPositionColor v2(Vector3(600.f, 450.f, 0.f), Colors::Yellow);
VertexPositionColor v3(Vector3(200.f, 450.f, 0.f), Colors::Yellow);

m_batch->DrawTriangle(v1, v2, v3);

m_batch->End();

Build and run to get the same image, but if you resize the window the triangle will not change in the second version if your window size is 800 by 600.

Technical notes

The BasicEffect family of shader classes uses shader code built in to the DirectXTK.lib as static data so there's no need to compile shaders at runtime or to load data files from disk.
Internally, both SpriteBatch and PrimitiveBatch make use of a dynamic rather than static vertex buffer object which makes use of special memory shared between the CPU and GPU. Generally, we prefer when possible to use static vertex buffers as they can reside in the video memory directly that is only accessible by the GPU.
Be sure to call SetVertexColorEnabled before calling GetVertexShaderBytecode as this impacts which shader permutation is actually used for rendering. See IEffect for more details.
Since we haven't used any textures yet, we don't need to use any sampler state objects.

State Objects

The use of CullNone for our rasterizer state above allows triangles and quads--which in Direct3D are just two triangles--to be drawn with arbitrary winding order. If you modify Render above as follows:

m_d3dContext->RSSetState( m_states->CullClockwise() );

Then build & run you run you will see nothing drawn because the triangle winding order was specified in clockwise order. If you changed it again to:

m_d3dContext->RSSetState( m_states->CullCounterClockwise() );

Then build & run you will see the triangle reappear.

For 'closed' objects, you typically use backface culling to speed up rendering which can quickly reject triangles that are not facing the viewer and avoids the need to run the pixel shader for those pixels.

The culling mode does not affect points or lines.

Drawing with textures

Start by saving rocks.jpg into your project's directory, and then from the top menu select Project / Add Existing Item.... Select "rocks.jpg" and click "OK".

In the Game.h file, add the following variable to the bottom of the Game class's private declarations:

Microsoft::WRL::ComPtr<ID3D11ShaderResourceView> m_texture;

In Game.cpp, add to the TODO of CreateDevice:

DX::ThrowIfFailed(
    CreateWICTextureFromFile(m_d3dDevice.Get(), L"rocks.jpg", nullptr,
    m_texture.ReleaseAndGetAddressOf()));

In Game.cpp, add to the TODO of OnDeviceLost:

m_texture.Reset();

Build and run to make sure the texture loads fine. Nothing else should be different yet.

Now go back to your Game.h and modify the VertexType alias we used earlier to use VertexPositionTexture.

using VertexType = DirectX::VertexPositionTexture;

Then in Game.cpp modify CreateDevice:

m_states = std::make_unique<CommonStates>(m_d3dDevice.Get());

m_effect = std::make_unique<BasicEffect>(m_d3dDevice.Get());
m_effect->SetTextureEnabled(true);
m_effect->SetTexture(m_texture.Get()); // Make sure you called CreateWICTextureFromFile before this point!

void const* shaderByteCode;
size_t byteCodeLength;

...

In Game.cpp, modify the TODO of Render:

m_d3dContext->OMSetBlendState( m_states->Opaque(), nullptr, 0xFFFFFFFF );
m_d3dContext->OMSetDepthStencilState( m_states->DepthNone(), 0 );
m_d3dContext->RSSetState( m_states->CullNone() );

m_effect->Apply(m_d3dContext.Get());

auto linear = m_states->LinearClamp();
m_d3dContext->PSSetSamplers(0, 1, &linear);

m_d3dContext->IASetInputLayout(m_inputLayout.Get());

m_batch->Begin();

VertexPositionTexture v1(Vector3(400.f, 150.f, 0.f), Vector2(.5f, 0));
VertexPositionTexture v2(Vector3(600.f, 450.f, 0.f), Vector2(1, 1));
VertexPositionTexture v3(Vector3(200.f, 450.f, 0.f), Vector2(0, 1));

m_batch->DrawTriangle(v1, v2, v3);

m_batch->End();