Thank you for the quick answer,
I've got it almost the way I want it to. The particle changes the color corresponding to its position in the background but a simple study shows a weird result.
See the pictures...
If the particle moves horizontal the color changes in the moving
direction, that is correct, but if the particle moves vertical, the
color change is contrary to the direction.My setup:
The background, in this example the checkerboard (green, blue, yellow, red), has a dimension of
500x500 and its centered in the origin of the scene (x=0,y=0,z=0).
The left bottom corner is in (x=-250, y=-250).
The top right corner is in (x=250, y=250).
The particle's dimension is 30x30 and its moving clock wise.
The application updates the position of the particle every frame, so the shader knows can
calculate the relative position in the background texture.
Shader (Cg):
//// UN-TWEAKABLES - AUTOMATICALLY-TRACKED TRANSFORMS ////////////////float4x4 WorldViewProj : WorldViewProjection;texture ObjectTexture < string UIName = "ObjectTexture"; string ResourceName = "../media/particle.png"; string ResourceType = "2D";>;sampler2D ObjectT : register(s0) = sampler_state { Texture = <ObjectTexture>;};texture BackgroundTexture < string UIName = "BackgroundTexture"; string ResourceName = "../media/background.png"; string ResourceType = "2D";>;sampler2D BackgroundT : register(s1) = sampler_state { Texture = <BackgroundTexture>;};/* data from application vertex buffer */struct appdata { float3 Position : POSITION; float4 UV : TEXCOORD0;};struct vertexOutput { float4 HPosition : POSITION; float4 UV : TEXCOORD0;};vertexOutput mainVS(appdata IN){ vertexOutput OUT; float4 Po = float4(IN.Position.xyz,1); OUT.HPosition = mul(WorldViewProj, Po); OUT.UV = IN.UV; return OUT;}float4 mainPS( vertexOutput IN, uniform float3 position, uniform float2 dimension) : COLOR{ float2 pos = 0; float2 size = 30; float2 originPos = position.xy - size*0.5; pos.x = (originPos.x + IN.UV.x * size.x) / dimension.x; pos.y = (originPos.y + IN.UV.y * size.y) / dimension.y; float4 particleColor = tex2D(ObjectT, IN.UV.xy); float4 backgroundColor = tex2D(BackgroundT, pos); float3 Color = (particleColor.rgb * particleColor.a) + (backgroundColor.rgb * (1.0 - particleColor.a)); float4 oColor = 0; oColor.rgb = Color.rgb; oColor.a = particleColor.a; return oColor;}technique technique1 { pass p0 { CullFaceEnable = false; VertexProgram = compile vp30 mainVS(); FragmentProgram = compile fp30 mainPS(position, dimension); }}
Do you have any idea why this behaves like this?
What I want to achieve is that the moving object gets coloured like the background
texture at the given position of the moving object.
I want to do this with a shader, because if it comes the colour pixels,
it's the right place to go. My prefered shader language is CG but if you familial with any other I'll be able to adapt it.
What I do know is the position of the moving object the position and
the dimension of the background texture.
Maybe I'm trying to do something very simple because I can't find
anything in the web, or I'm using the wrong buzz words.
This is actually a workaround for my original problem that I have:
I'm using a particle system where the particle is a plane object and has a translucent circle
with a white border as texture, so you can see the background through it.
And the more particle there are the brighter the white will get.
That behavior is right but not realy wanted. Because when the number
of the particle went through the roof, I end up with an opaque white
and the particles form a spongy mass and you can't see any more details in it.
Is there an apha blending mode that I missed that will do the job?
I'm using "add color" right now ...
One way to solve this problem is to make the center of particle texture opaque,
but I'm using the background texture as static light source so the particle gets
brighter at the top of the scene and darker at the bottom.
Thank you for your help.
If you need more details please ask.
