I am following this tutorials series on rastertek.com and I got a bit stuck on the Diffuse Lighting Tutorial. Particulary, the part that does not work for me is alpha channel of the light color. Here are the [I think they are] relevant code parts:

Light Shader:

cbuffer MatrixBuffer
    matrix worldMatrix;
    matrix viewMatrix;
    matrix projectionMatrix;

Texture2D shaderTexture;
SamplerState SampleType;

cbuffer LightBuffer
    float4 ambientColor;
    float4 diffuseColor;
    float3 lightDirection;
    float padding;

struct PixelInputType
    float4 position : SV_POSITION;
    float2 tex : TEXCOORD0;
    float3 normal : NORMAL;

float4 LightPixelShader(PixelInputType input) : SV_TARGET
    float4 textureColor;
    float3 lightDir;
    float lightIntensity;
    float4 color;

    textureColor = shaderTexture.Sample(SampleType, input.tex);

    lightDir = -lightDirection;

    lightIntensity = saturate(dot(input.normal, lightDir));

    color = saturate(diffuseColor * lightIntensity);

    color = color * textureColor;

    return color;

PixelInputType LightVertexShader(VertexInputType input)
    PixelInputType output;

    input.position.w = 1.0f;

    output.position = mul(input.position, worldMatrix);
    output.position = mul(output.position, viewMatrix);
    output.position = mul(output.position, projectionMatrix);

    output.tex = input.tex;

    output.normal = mul(input.normal, (float3x3)worldMatrix);

    output.normal = normalize(output.normal);

    return output;

The problem here is the shader completely ignores the alpha channel of the light. If I set the diffuse color to red, then the texture will be reddish no matter what alpha is supplied.

The only way I could make the alpha work is changing the line

color = color * textureColor;


color = color * color.w * textureColor;

in the pixel shader. This however gives overall wrong result.

Might it be the DirectX setup issue, here are some parts of the device initialization:

// Set regular 32-bit surface for the back buffer.
swapChainDesc.BufferDesc.Format = DXGI_FORMAT_R8G8B8A8_UNORM;


// Set up the description of the depth buffer.
depthBufferDesc.Width = screenWidth;
depthBufferDesc.Height = screenHeight;
depthBufferDesc.MipLevels = 1;
depthBufferDesc.ArraySize = 1;
depthBufferDesc.Format = DXGI_FORMAT_D24_UNORM_S8_UINT;
depthBufferDesc.SampleDesc.Count = 1;
depthBufferDesc.SampleDesc.Quality = swapChainDesc.SampleDesc.Quality;
depthBufferDesc.Usage = D3D11_USAGE_DEFAULT;
depthBufferDesc.BindFlags = D3D11_BIND_DEPTH_STENCIL;
depthBufferDesc.CPUAccessFlags = 0;
depthBufferDesc.MiscFlags = 0;


// Setup the raster description which will determine how and what polygons will be drawn.
rasterDesc.AntialiasedLineEnable = false;
rasterDesc.CullMode = D3D11_CULL_BACK;
rasterDesc.DepthBias = 0;
rasterDesc.DepthBiasClamp = 0.0f;
rasterDesc.DepthClipEnable = true;
rasterDesc.FillMode = D3D11_FILL_SOLID;
rasterDesc.FrontCounterClockwise = false;
rasterDesc.MultisampleEnable = false;
rasterDesc.ScissorEnable = false;
rasterDesc.SlopeScaledDepthBias = 0.0f;

What's wrong with this shader?

I'm not sure what other code might be relevant, I will gladly add any at your request.

  • \$\begingroup\$ What effect would you expect / want an alpha value of a light to have? \$\endgroup\$ – DMGregory Feb 12 at 10:20
  • \$\begingroup\$ @DMGregory to affect the light intensity, i.e. if I set my diffuse light to (1.0, 1.0, 1.0, 0.2), the textured model will only have 20% lightness as if the light source was weak. But my understand of diffuse light might be wrong... \$\endgroup\$ – lolbas Feb 12 at 11:09
  • \$\begingroup\$ But you already have a dedicated light intensity variable, in addition to the brightness/darkness of the light's RGB color itself. Why add a third variable to control the same thing? \$\endgroup\$ – DMGregory Feb 12 at 11:16
  • \$\begingroup\$ @DMGregory but isn't lightIntensity is for intensity across the polygon, not for the intensity of the light source? Or do you mean I can multiply it bydiffuseColor.w to get the result I describe? I actually haven't tried this way and couldn't test it soon but it sounds.. reasonable? \$\endgroup\$ – lolbas Feb 12 at 11:28
  • \$\begingroup\$ Ah, sorry, that was a misreading on my part (on mobile so I only see part of the code at a time). I thought it was part of your light buffer rather than a name for your Lambertian / diffuse illumination / n dot l term. \$\endgroup\$ – DMGregory Feb 12 at 11:30

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