I am trying get 3 new random floats into my pixel shader for each pixel. Based on what I have read here, here, and also here, I believe that I need to generate a large texture containing random RGB values and then during each draw call randomly generate a couple of texture coordinate offset values to produce a pseudo-random effect. Is the only way to do this through the LockRect and UnlockRect API? I hope not. Also when do the floating point color values get quantized. For example if I do a ColorFill on a texture and set it too a RGB of ( 0.52, 0.52, 0.52 )
which on a 0-255 scale is ( 132.6, 132.6, 132. 6 ) when I access that texture in my pixel shader will I get a value of 0.52 or 0.52156....i.e. (133).

You may create a texture with random numbers by locking the texture and filling it with CPU generate random values.
If you are just after noise, you can precreate a texture (in photoshop or similar program) containing noise.

The values you can store in the texture depends on the surface format. RGBA8 format for example contains values 0-255 for each channel which maps to range [0..1]. RGBA8 texture can contain only integer values, which means that value stored for 0.52 should be multiplied by 255 before storing in the texture and it will result 132 or 133 depending on rounding. The value that the shader read from the texture is 133/255=0,521568...

You may use another texture format to get more precision if required.

Cheers!

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 too if you want to look:


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