请求修改MAME中CRT磷光轨迹HLSL着色器，降低轨迹亮度50%

问题说明 #

我正在修改MAME模拟器的CRT磷光轨迹（phosphor trails）HLSL着色器，目标是把磷光轨迹的像素亮度降低50%。作为HLSL新手，我尝试调整颜色参数后，发现修改只影响了轨迹的持续时间，没达到预期的亮度调整效果，求正确修改方式。

原着色器代码 #

// license:BSD-3-Clause
// copyright-holders:Ryan Holtz
//-----------------------------------------------------------------------------
// Phosphor Effect
//-----------------------------------------------------------------------------

//-----------------------------------------------------------------------------
// Sampler Definitions
//-----------------------------------------------------------------------------

texture Diffuse;

sampler DiffuseSampler = sampler_state
{
    Texture   = <Diffuse>;
    MipFilter = LINEAR;
    MinFilter = LINEAR;
    MagFilter = LINEAR;
    AddressU = CLAMP;
    AddressV = CLAMP;
    AddressW = CLAMP;
};

texture LastPass;

sampler PreviousSampler = sampler_state
{
    Texture   = <LastPass>;
    MipFilter = LINEAR;
    MinFilter = LINEAR;
    MagFilter = LINEAR;
    AddressU = CLAMP;
    AddressV = CLAMP;
    AddressW = CLAMP;
};

//-----------------------------------------------------------------------------
// Vertex Definitions
//-----------------------------------------------------------------------------

struct VS_OUTPUT
{
    float4 Position : POSITION;
    float4 Color : COLOR0;
    float2 TexCoord : TEXCOORD0;
    float2 PrevCoord : TEXCOORD1;
};

struct VS_INPUT
{
    float3 Position : POSITION;
    float4 Color : COLOR0;
    float2 TexCoord : TEXCOORD0;
};

struct PS_INPUT
{
    float4 Color : COLOR0;
    float2 TexCoord : TEXCOORD0;
    float2 PrevCoord : TEXCOORD1;
};

//-----------------------------------------------------------------------------
// Phosphor Vertex Shader
//-----------------------------------------------------------------------------

uniform float2 ScreenDims;
uniform float2 TargetDims;

uniform bool Passthrough;

VS_OUTPUT vs_main(VS_INPUT Input)
{
    VS_OUTPUT Output = (VS_OUTPUT)0.0;

    Output.Position = float4(Input.Position.xyz, 1.0);
    Output.Position.xy /= ScreenDims;
    Output.Position.y = 1.0 - Output.Position.y; // flip y
    Output.Position.xy -= 0.5; // center
    Output.Position.xy *= 2.0; // zoom

    Output.TexCoord = Input.TexCoord;
    Output.TexCoord += 0.5 / TargetDims; // half texel offset correction (DX9)

    Output.PrevCoord = Output.TexCoord;

    Output.Color = Input.Color;

    return Output;
}

//-----------------------------------------------------------------------------
// Phosphor Pixel Shader
//-----------------------------------------------------------------------------

uniform float DeltaTime = 0.0;
uniform float3 Phosphor = float3(0.0, 0.0, 0.0);

static const float F = 30.0;

float4 ps_main(PS_INPUT Input) : COLOR
{
    float4 CurrY = tex2D(DiffuseSampler, Input.TexCoord);
    float3 PrevY = tex2D(PreviousSampler, Input.PrevCoord).rgb;

    PrevY[0] *= Phosphor[0] == 0.0 ? 0.0 : pow(Phosphor[0], F * DeltaTime);
    PrevY[1] *= Phosphor[1] == 0.0 ? 0.0 : pow(Phosphor[1], F * DeltaTime);
    PrevY[2] *= Phosphor[2] == 0.0 ? 0.0 : pow(Phosphor[2], F * DeltaTime);
    float a = max(PrevY[0], CurrY[0]);
    float b = max(PrevY[1], CurrY[1]);
    float c = max(PrevY[2], CurrY[2]);
    return Passthrough ? CurrY : float4(a, b, c, CurrY.a);
}

//-----------------------------------------------------------------------------
// Phosphor Technique
//-----------------------------------------------------------------------------

technique DefaultTechnique
{
    pass Pass0
    {
        Lighting = FALSE;

        VertexShader = compile vs_2_0 vs_main();
        PixelShader  = compile ps_2_0 ps_main();
    }
}

我的错误尝试 #

我修改了像素着色器中的混合逻辑，试图降低轨迹亮度，但这反而影响了轨迹持续时间：

// Now blend, starting the trails at 45% brightness
float a = max(PrevY[0] * 0.45, CurrY[0]);
float b = max(PrevY[1] * 0.45, CurrY[1]);
float c = max(PrevY[2] * 0.45, CurrY[2]);

正确修改方式 #

问题出在你的修改位置：你在max函数里对PrevY乘系数，相当于每帧都额外衰减一次轨迹亮度，导致轨迹消失速度变快。要单独降低轨迹亮度且不影响持续时间，应该在轨迹衰减计算完成后、与当前帧混合前，对PrevY整体乘0.5。

修改后的像素着色器核心代码如下：

float4 ps_main(PS_INPUT Input) : COLOR
{
    float4 CurrY = tex2D(DiffuseSampler, Input.TexCoord);
    float3 PrevY = tex2D(PreviousSampler, Input.PrevCoord).rgb;

    // 原有的轨迹衰减计算（控制持续时间，不要修改）
    PrevY[0] *= Phosphor[0] == 0.0 ? 0.0 : pow(Phosphor[0], F * DeltaTime);
    PrevY[1] *= Phosphor[1] == 0.0 ? 0.0 : pow(Phosphor[1], F * DeltaTime);
    PrevY[2] *= Phosphor[2] == 0.0 ? 0.0 : pow(Phosphor[2], F * DeltaTime);

    // 新增：将轨迹亮度降低50%
    PrevY *= 0.5;

    // 原有的混合逻辑，保持不变
    float a = max(PrevY[0], CurrY[0]);
    float b = max(PrevY[1], CurrY[1]);
    float c = max(PrevY[2], CurrY[2]);
    return Passthrough ? CurrY : float4(a, b, c, CurrY.a);
}

原理说明 #

• 原代码中pow(Phosphor[0], F * DeltaTime)是控制磷光轨迹衰减速率的核心，决定了轨迹的持续时间，这个部分不需要改动。
• 在衰减计算完成后对PrevY乘0.5，只会降低当前帧轨迹的亮度，不会改变每帧的衰减比例，因此轨迹的持续时间不会受影响，完美达成降低轨迹亮度50%的目标。

内容的提问来源于stack exchange，提问作者vespworks