Blending
Outline
• The Blending Equation
• Blend Factors
• Alpha Test
WKTai/GAME Lab.
Blending
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Introduction
• Blending
– A technique to blend (combine) pixels that we are
currently rasterizing with pixels that have been
previously rasterized to the same pixel locations
• Demo TeapotDemo.exe
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The Blending Equation
• Variables are a 4D color vector (r, g, b, a)
•
denotes component-wise multiplication
• OutputPixel:
OutputPixel The resulting blended pixel
• SourcePixel
– The pixel currently being computed that is to be blended with
the pixel on the back buffer
• SourceBlendFactor
– A value in the interval [0, 1] that specifies the percent of the
source pixel to use in the blend
• DestPixel:
DestPixel The pixel currently on the back buffer
• DestBlendFactor
– A value in the interval [0, 1] that specifies the percent of the
destination pixel to use in the blend
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“+” Operator in the Blending Function
• “+” operator can be changed by setting the
D3DRS_BLENDOP
gd3dDevice->SetRenderState(D3DRS_BLENDOP, ???);
???
• D3DBLENDOP
– Defines the supported blend operations
typedef enum D3DBLENDOP
{
D3DBLENDOP_ADD = 1,
D3DBLENDOP_SUBTRACT = 2,
D3DBLENDOP_REVSUBTRACT = 3,
D3DBLENDOP_MIN = 4,
D3DBLENDOP_MAX = 5,
D3DBLENDOP_FORCE_DWORD = 0x7fffffff,
} D3DBLENDOP, *LPD3DBLENDOP;
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D3DBLENDOP
• D3DBLENDOP_ADD
– OutputPixel = SourcePixel⊗SourceBlendFactor +
DestPixel⊗DestBlendFactor
• D3DBLENDOP_SUBTRACT
– OutputPixel = SourcePixel⊗SourceBlendFactor DestPixel⊗DestBlendFactor
• D3DBLENDOP_REVSUBTRACT
– OutputPixel = DestPixel⊗DestBlendFactor SourcePixel⊗SourceBlendFactor
• D3DBLENDOP_MIN
– OutputPixel = MIN(SourcePixel⊗SourceBlendFactor,
MIN
DestPixel⊗DestBlendFactor)
• D3DBLENDOP_MAX
– OutputPixel = MAX(SourcePixel⊗SourceBlendFactor,
MAX
DestPixel⊗DestBlendFactor)
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Outline
•
•
•
The Blending Equation
Blend Factors
Alpha Test
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Blending
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Blend Factors
• Setting different combinations of source and
destination blend factors
– Device->SetRenderState(D3DRS_SRCBLEND,
SRC
Source);
– Device->SetRenderState(D3DRS_DESTBLEND,
DEST
Destination);
• Source and Destination can be one of the following
blend factors:
– D3DBLEND_ZERO—blendFactor=(0, 0, 0, 0)
– D3DBLEND_ONE—blendFactor=(1, 1, 1, 1)
– D3DBLEND_SRCCOLOR—blendFactor=(r
SRC
s, gs, bs, as)
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Blend Factors (Contd.)
• D3DBLEND_INVSRCCOLOR
– blendFactor=(1 – rs, 1 – gs, 1 – bs,1 – as)
• D3DBLEND_SRCALPHA
// default
– blendFactor=(αs, αs, αs, αs)
• D3DBLEND_INVSRCALPHA
// default
– blendFactor=(1 – αs, 1 – αs, 1 – αs,1 – αs)
• D3DBLEND_DESTALPHA
– blendFactor=(αd, αd, αd, αd)
• D3DBLEND_INVDESTALPHA
– blendFactor=(1 – αd, 1 – αd, 1 – αd,1 – αd)
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Blend Factors (Contd.)
• D3DBLEND_DESTCOLOR
DEST
– blendFactor=(rd, gd, bd, αd)
• D3DBLEND_INVDESTCOLOR
DEST
– blendFactor=(1 – rd, 1 – gd, 1 – bd,1 – αd)
• D3DBLEND_SRCALPHASAT
SRC
– blendFactor=(f, f, f, 1), where f=min(αs, 1 – αd)
• D3DBLEND_BOTHINVSRCALPHA
SRC
– Sets the source blend factor to (1–αs, 1–αs, 1–αs, 1–αs)
and
– The destination blend factor to (αs, αs, αs, αs)
– This blend mode is only valid for D3DRS_SRCBLEND
SRC
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Blending
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Blend Factors (Contd.)
• Set the source and destination blend factors in an effect
file inside a pass block
Pass p0
{
vertexShader= compile vs_2_0 VX();
pixelShader= compile ps_2_0 PS();
AlphaBlendEnable=
AlphaBlendEnable true;
SrcBlend= One;
DestBlend= One;
BlendOp= RevSubtract;
}
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Blend Factors (Contd.)
Pass p0
{
vertexShader= compile vs_2_0 VX();
pixelShader= compile ps_2_0 PS();
// use these states to blend RGB components
AlphaBlendEnable=
AlphaBlendEnable true;
SrcBlend= One;
DestBlend= One;
BlendOp= RevSubtract;
// Blend the alpha component
SeparateAlphaBlendEnable
= ture;
Alpha
SrcBlendAlpha = ScrAlpha;
DestBlendAlpha
= InvScrAlpha;
Dest
BlendOpAlpha = Add;
}
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Blend Factor Example
• Keep the orignial destination pixel exactly as it is
– Not overwrite or blend it with the source pixel currently
being rasterized
• Set the source pixel blend factor to
D3DBLEND_ZERO and
the destination pixel blend factor to
D3DBLEND_ONE
OutputPixel= SourcePixel ⊗ SourceBlendFactor +
DestPixel ⊗ DestBlendFactor
= SourcePixel ⊗ (0,0,0,0) + DestPixel ⊗(1,1,1,1)
= (0,0,0,0) + DestPixel
= DestPixel
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Blending
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Code Snippet
gd3dDevice->SetRenderState(
D3DRS_BLENDOP, D3DBLENDOP_ADD);
ADD
Device->SetRenderState(
D3DRS_SRCBLEND, D3DBLEND_ZERO);
D3DBLEND_ZERO
Device->SetRenderState(
D3DRS_DESTBLEND, D3DBLEND_ONE);
ONE
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Blend Factor Example II
• Blending by adding two images
OutputPixel= SourcePixel ⊗ SourceBlendFactor +
DestPixel ⊗ DestBlendFactor
= SourcePixel ⊗ (1,1,1,1) + DestPixel ⊗(1,1,1,1)
= SourcePixel + DestPixel
• Code snippet
gd3dDevice->SetRenderState(
D3DRS_BLENDOP, D3DBLENDOP_ADD);
ADD
Device->SetRenderState(
D3DRS_SRCBLEND, D3DBLEND_ONE);
D3DBLEND_ONE
Device->SetRenderState(
D3DRS_DESTBLEND, D3DBLEND_ONE);
ONE
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Blend Factor Example III
• Multiply a source pixel with its corresponding
destination pixel
OutputPixel= SourcePixel ⊗ SourceBlendFactor +
DestPixel ⊗ DestBlendFactor
= SourcePixel ⊗ (0,0,0,0) + DestPixel ⊗ SourcePixel
= DestPixel ⊗ SourcePixel
• Code snippet
gd3dDevice->SetRenderState(
D3DRS_BLENDOP, D3DBLENDOP_ADD);
ADD
Device->SetRenderState(
D3DRS_SRCBLEND, D3DBLEND_ZERO);
D3DBLEND_ZERO
Device->SetRenderState(
D3DRS_DESTBLEND, D3DBLEND_SRCCOLOR);
SRCCOLOR
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Blend Factor Example III
• Similar to texture modulation for lightmap
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Blend Factor Example IV
• Blend the source and destination pixels based
on the transparency percent of the source pixel
– Let the source alpha component as be thought of as
α percent that controls the transparency of the
source pixel
OutputPixel= SourcePixel ⊗ SourceBlendFactor +
DestPixel ⊗ DestBlendFactor
= SourcePixel ⊗ (sα, sα, sα, sα) + DestPixel⊗(1sα, 1-sα, 1-sα, 1-sα)
= SourcePixel sα ⊗DestPixel (1-sα )
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Code snippet
gd3dDevice->SetRenderState(
D3DRS_BLENDOP, D3DBLENDOP_ADD);
ADD
Device->SetRenderState(
D3DRS_SRCBLEND, D3DBLEND_SRCALPHA);
D3DBLEND_SRCALPHA
Device->SetRenderState(
D3DRS_DESTBLEND,
D3DBLEND_INVSRCALPHA);
SRCALPHA
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Blending
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Enable/Disable Blending Operation
• Enable by setting the
D3DRS_ALPHABLENDENABLE render state
to true
– Blending is disabled by default
Device->SetRenderState(
RenderState
D3DRS_ALPHABLENDENABLE, true);
• Disable blending
Device->SetRenderState(
RenderState
D3DRS_ALPHABLENDENABLE, false);
WKTai/GAME Lab.
Blending
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Note on Render State
• To control which color components the graphics
device writes to
– Set D3DPMISCCAPS_COLORWRITEENABLE
bit in
WRITE
D3DCAPS9::PrimitiveMiscCaps
• Example
– To write only to the green and blue channels
– IDirect3DDevice9::SetRenderState(
D3DRS_COLORWRITEENABLE,
WRITE
D3DCOLORWRITEENABLE_BLUE |
D3DCOLORWRITEENABLE_GREEN
);
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Note on Render State (Contd.)
• Default setting
– Writes to all four color channels
– IDirect3DDevice9::SetRenderState(
D3DRS_COLORWRITEENABLE,
WRITE
D3DCOLORWRITEENABLE_ALPHA |
D3DCOLORWRITEENABLE_BLUE |
D3DCOLORWRITEENABLE_GREEN |
D3DCOLORWRITEENABLE_RED);
RED
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Rules When Using Blending
1. Draw objects that do not use blending first
2. Then sort the objects that use blending by their
distance from the camera
3. Draw the objects that use blending in a back-tofront order
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Blending
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Demo Teapot Demo.exe
• Demo TeapotDemo.exe
– Use the source pixel’s alpha component to control its
transparency
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TeapotDemo.exe
void TeapotDemo::drawTeapot()
{ // Cylindrically interpolate texture coordinates.
HR(gd3dDevice->SetRenderState(
D3DRS_WRAP0, D3DWRAPCOORD_0));
// Enable alpha blending.
HR(gd3dDevice->SetRenderState(
D3DRS_ALPHABLENDENABLE,
true));
ALPHA
true
HR(gd3dDevice->SetRenderState(
D3DRS_SRCBLEND,
D3DBLEND_SRCALPHA));
SRC
SRCALPHA
HR(gd3dDevice->SetRenderState(
D3DRS_DESTBLEND,
BLEND D3DBLEND_INVSRCALPHA));
INV
…
// Disable alpha blending.
HR(gd3dDevice>SetRenderState(D3DRS_ALPHABLENDENABLE,
false));
ALPHA
false
}
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TeapotDemo.exe
• Setting the diffuse material’s alpha component
TeapotDemo::TeapotDemo(
{
…
mTeapotMtrl.ambient = D3DXCOLOR(1.0f, 1.0f, 1.0f, 1.0f);
1.0f
mTeapotMtrl.diffuse = D3DXCOLOR(1.0f, 1.0f, 1.0f, 0.5f);
0.5f
mTeapotMtrl.spec
= D3DXCOLOR(0.8f, 0.8f, 0.8f, 1.0f);
1.0f
mTeapotMtrl.specPower = 16.0f;
…
}
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TeapotDemo.exe
• In vertex shader,
shader just pass along the alpha
component to the pixel shader
uniform extern float4 gDiffuseMtrl;
gDiffuseMtrl
struct OutputVS
…
{
float4 posH : POSITION0;
float4 diffuse : COLOR0;
COLOR0
float4 spec : COLOR1;
float2 tex0 : TEXCOORD0;
OutputVS DirLightTexVS(…)
{
…
};
outVS.diffuse.
a = gDiffuseMtrl.
outVS
gDiffuseMtrl a;
…
return outVS;
outVS
}
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TeapotDemo.exe
• In pixel shader,
shader set the pixel’s alpha value to the
interpolated color’s alpha component
float4 DirLightTexPS(float4 c : COLOR0,
COLOR0
float4 spec : COLOR1,
float2 tex0 : TEXCOORD0) : COLOR
{
float3 texColor = tex2D(TexS, tex0).rgb;
float3 diffuse = c.rgb * texColor; // modulation
return float4(diffuse + spec.rgb, c.a);
}
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Blending
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Control Transparency by Texture Alpha
Channel
• Using material’s alph, the level of transparency
will be uniformly set for the entire triangles
• Using texture alpha channel, control the
transparency at the pixel level
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Blending
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Creating Textures with Alpha Channel
• Create the alpha channel by
– Adobe Photoshop
– Using DXTex utility program
• Steps
– DirectX Texture Tool -> open -> brick.jpg (24-bit RGB texture,
D3DFMT_R8G8B8)
*Change format to support an alpha channel
• 32-bit ARGB texture format D3DFMT_A8R8G8B8
• Or a compressed format, like D3DFMT_DXT3
– Format -> Change Surface Format -> DXT3
– Load the 8-bit grayscale
– File-> Open Onto Alpha Channel Of This Texture -> alpha.jpg
– #
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Blending
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Brick Texture with Alpha Channel
RGB Textrue
Alpha grayscale
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Transparent Teapot Demo
• Demo Teapot with TexAlpha.exe
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Code Snippet for Pixel Shader
float4 DirLightTexPS(float4 c : COLOR0, float4
spec : COLOR1, float2 tex0 : TEXCOORD0) :
COLOR
{
float4 texColor = tex2D(TexS, tex0);
float3 diffuse = c.rgb * texColor.rgb;
return float4(diffuse + spec.rgb, texColor.a*c.a);
texColor.a
}
• When using an alpha channel, usually set the
diffuse material’s alpha component to 1.0
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Outline
• The Blending Equation
• Blend Factors
• Alpha Test
WKTai/GAME Lab.
Blending
34
Alpha Test
• To discard a pixel (not render it) based on its
alpha value in comparison to some reference
value
if αs ⊙ ref == true then accept pixel
else reject pixel
αs :Source alpha component
ref :Application-defined reference value
⊙ :Is an operation by D3DCMPFUNC enumerated
type
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D3DCMPFUNC
typedef enum D3DCMPFUNC
{
D3DCMP_NEVER = 1, // Always fail the test
D3DCMP_LESS = 2,
D3DCMP_EQUAL = 3,
D3DCMP_LESSEQUAL = 4,
D3DCMP_GREATER = 5,
D3DCMP_NOTEQUAL = 6,
D3DCMP_GREATEREQUAL = 7,
D3DCMP_ALWAYS = 8,
// Always pass the test
D3DCMP_FORCE_DWORD = 0x7fffffff,
} D3DCMPFUNC, *LPD3DCMPFUNC;
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Example Code
• Enable the alpha test, set the ref= 100, and set the
comparison operation to
D3DCMP_GREATERQUAL
– Only pixels with alpha values >= 100 are draw
HR(gd3dDevice->SetRenderState(
D3DRS_ALPHATESTENABLE, true));
true
HR(gd3dDevice->SetRenderState(
D3DRS_ALPHAFUNC,
D3DRS_ALPHAFUNC D3DCMP_GREATEREQUAL));
HR(gd3dDevice->SetRenderState(
D3DRS_ALPHAREF,
REF 100));
100
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Alpha Test in Effect File
Pass P0
{
vertexShader = compile vs_2_0 VS();
pixelShader = compile ps_2_0 PS();
AlphaTestEnable = true;
AlphaFunc = GreaterEqual;
AlphaRef = 220;
}
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Application of Alpha Test
• A quad with texture and
alpha test for saving
polygons
• Demo GateDemo.exe
• Any other applications?
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Blending
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Remarks
• The Blending Equation
• Blend Factors
• Alpha Test
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