Why is this 128Bit Color Format being converted to 32Bit

问题

I am trying to write an HLSL pixel shader for a project I am working on. Basically want I want to do is if a texture has a pixel with a float value of 0.52 (on scale of 0-255 is 132.6) I want to output 133 60% of the time and output 132 40% of the time. Write now I am just trying to output the fractional remainder of the RGB value (i.e. the chance to bump the pixel up) however I always get a value of zero) I think this is because the colors are getting quantized to a 0-255 scale before they reach the shader but I don't know why this would be because I am useing the A32B32G32R32f format which should be able to store plenty of information about the colors. Here is my very simple shader code. I am using SlimDX (DX9) if that matters.

sampler2D Tex0 : register(s0);
float4 DitherByChance(float2 coords : TEXCOORD0) : COLOR
{
    float4 newColor = float4(0, 0, 0, 0);   // The pixel color to return
    double4 oldColor = tex2D(Tex0, coords);
    double scale = 0.0039215686274509803921568627451;  // 1 / 255

    double rPercentChance = frac(oldColor.r / scale);   //Chance to round red channel up
    double gPercentChance = frac(oldColor.g / scale);   //Chance to round green channel up
    double bPercentChance = frac(oldColor.b / scale);   //Chance to round blue channel up

    newColor.r = rPercentChance;
    newColor.g = gPercentChance;
    newColor.b = bPercentChance;

    newColor.a = 1;

    return newColor;
}

technique DitherViaChance  
{  
    pass Pass1  
    {  
        PixelShader = compile ps_2_0 DitherByChance();      
    }  
}

Here is my slimDX code if it matters:

Public Class TextStimulusDisplay
    Inherits BaseStimulusDisplay

    Protected ditherByChanceEffect As PixelShader
    Protected psByteCode As ShaderBytecode
    Protected gStream As DataStream
    Protected ditherBC As Effect

    Protected tex As Texture
    Protected spriteRender As Sprite

    Public displaySettings As DisplaySettings
    Public charText As Texture
    Public randomText As Texture

    Protected rando As Random

    Public Sub New(ByVal displaysettings As DisplaySettings, Optional ByVal WindowedMode As Boolean = False)
        MyBase.New(New Size(displaysettings.xResolution, displaysettings.yResolution), WindowedMode)
        Me.displaySettings = displaysettings
    End Sub

    Public Overrides Sub CreateDeviceResources()
        MyBase.CreateDeviceResources()
        ditherBC = Effect.FromFile(device, Application.StartupPath & "\effects\DitherByChance.fx", ShaderFlags.Debug)
        rando = New Random()

        randomText = New Texture(device, 100, 100, 1, Usage.Dynamic, Format.A32B32G32R32F, Pool.Default)
        Dim data(80000) As Byte
        rando.NextBytes(data)
        Dim dataBox As DataRectangle = randomText.GetSurfaceLevel(0).LockRectangle(LockFlags.None)
        dataBox.Data.Seek(0, IO.SeekOrigin.Begin)
        dataBox.Data.Write(data, 0, data.Length)
        dataBox.Data.Close()
        randomText.GetSurfaceLevel(0).UnlockRectangle()

        device.SetTexture(1, randomText)

        'psByteCode = ShaderBytecode.CompileFromFile(Application.StartupPath & "\effects\DitherByChance.fx", "DitherByChance", "ps_2_0", ShaderFlags.None)
        'ditherByChanceEffect = New PixelShader(device, psByteCode)
        'device.PixelShader = ditherByChanceEffect
        spriteRender = New Sprite(device)
        spriteRender.Transform = Matrix.Identity
        tex = createCharacterTexture()

    End Sub

    Public Overrides Sub ReleaseDeviceResources()
        MyBase.ReleaseDeviceResources()
    End Sub

    Public Overrides Sub RenderScene()
        'ditherBC.Technique = ditherBC.GetTechnique("DitherViaChance")

        Me.device.BeginScene()
        spriteRender.Begin(SpriteFlags.None)
        ditherBC.Begin()
        ditherBC.BeginPass(0)
        spriteRender.Draw(tex, New Rectangle(0, 0, 50, 50), New Color4(1, 1, 1))
        'spriteRender.Draw(randomText, New Rectangle(0, 0, 1000, 1000), New Color4(1, 1, 1))
        spriteRender.End()
        ditherBC.EndPass()
        ditherBC.End()
        Me.device.EndScene()
    End Sub

    Public Function createCharacterTexture() As Texture
        Dim RtsHelper As RenderToSurface = New RenderToSurface(device, 100, 100, Format.A32B32G32R32F)
        Dim texture As Texture = New Texture(device, 100, 100, 1, Usage.RenderTarget, Format.A32B32G32R32F, Pool.Default)

        Dim sloanFont As Font = New Font(device, 98, 98, FontWeight.Normal, 1, False, CharacterSet.Default, Precision.Default,
                                         FontQuality.ClearTypeNatural, PitchAndFamily.Default, "Sloan")


        RtsHelper.BeginScene(texture.GetSurfaceLevel(0), New Viewport(0, 0, 100, 100))



        sloanFont.DrawString(Nothing, "A", 1, 1, New SlimDX.Color4())
        RtsHelper.EndScene(Filter.None)

        Font.Dispose()
        RtsHelper.Dispose()

        Return texture
    End Function
End Class

来源：https://stackoverflow.com/questions/12839758/why-is-this-128bit-color-format-being-converted-to-32bit