alpha value captured by camera in ios - ios

Question

i am trying to make a fake shadow by using RenderTexture
i dynamically create a camera, and assign a RenderTexture to the camera as render target.
the format of the RenderTexture is set to ARGB32
then i use the RenderTexture as texture on a plane with a custom shader to change the color to black and adjust the alpha value so the shadow can be properly shown
below is the shader im using:
Shader "Fake Shadow" {
Properties {
_ShadowTex ("Fake Shadow", 2D) = "white" { TexGen ObjectLinear }
_SAlpha ("Shadow Intensity", float) = 0.35
}
Category {
Tags { "Queue" = "Transparent-1" }
Lighting Off
ZWrite On
Cull Back
Blend SrcAlpha OneMinusSrcAlpha, one one
Subshader {
LOD 200
Pass {
SetTexture[_ShadowTex] {
ConstantColor(0,0,0,[_SAlpha])
matrix [_ProjMatrix]
Combine texture * constant, texture * constant
}
}
}
}
}
things are normal when i test play in Unity3D editor and on Android
however, the result is not correct in ios
the result image is in the link below (sorry i cant post image yet)
shadow in editor
shadow in ios
it seems ios put alpha as 1 in the RenderTexture even on pixels which the camera captured nothing
i have found a solution to solve this problem which is to check 32-bit display in the player settings
but im worried that checking 32-bit display will affect the performance
is there any other solution without checking 32-bit display?

Answer 1

I'm doing the same thing (camera and projector for emulating shadows), and I use a shader based on the one on this page: http://en.wikibooks.org/wiki/Cg_Programming/Unity/Projectors
The shader follows:
Shader "Cg projector shader for drop shadows" {
Properties {
_ShadowTex ("Projected Image", 2D) = "white" {}
_ShadowStrength ("Shadow Strength", Float) = 0.65
}
SubShader {
Pass {
// Blend Off
Blend Zero OneMinusSrcAlpha // attenuate color in framebuffer
// by 1 minus alpha of _ShadowTex
// fb_color = black + fb_color * OneMinusShadowAlpha;
CGPROGRAM
#pragma vertex vert
#pragma fragment frag
// User-specified properties
uniform sampler2D _ShadowTex;
uniform float _ShadowStrength;
// Projector-specific uniforms
uniform float4x4 _Projector; // transformation matrix
// from object space to projector space
struct vertexInput {
float4 vertex : POSITION;
float3 normal : NORMAL;
};
struct vertexOutput {
float4 pos : SV_POSITION;
float4 posProj : TEXCOORD0;
// position in projector space
};
vertexOutput vert(vertexInput input)
{
vertexOutput output;
output.posProj = mul(_Projector, input.vertex);
output.pos = mul(UNITY_MATRIX_MVP, input.vertex);
return output;
}
float4 frag(vertexOutput input) : COLOR
{
if (input.posProj.w > 0.0) // in front of projector?
{
return tex2D(_ShadowTex ,
float2(input.posProj) / input.posProj.w) * _ShadowStrength;
}
else // behind projector
{
return float4(0.0);
}
}
ENDCG
}
}
// The definition of a fallback shader should be commented out
// during development:
// Fallback "Projector/Light"
}