如何使着色器淡出一种颜色？

Question

This is the current shader I am using. It fades the object by slowly reducing the opacity. I want to fade to purple. How can this be done?

shader.frag:

uniform sampler2D texture;
uniform float opacity;

void main()
{
    vec4 pixel = texture2D(texture, gl_TexCoord[0].xy);
    gl_FragColor = pixel * vec4(1.0, 1.0, 1.0, opacity);
}

shader.vert:

void main()
{
    gl_Position = gl_ModelViewProjectionMatrix * gl_Vertex;
    gl_TexCoord[0] = gl_TextureMatrix[0] * gl_MultiTexCoord0;
    gl_FrontColor = gl_Color;
}

application of shader in main function:

sf::Shader shader;
if (!shader.loadFromFile("shader.vert", "shader.frag"))
    return EXIT_FAILURE;

float opacity = 1.0; //transparency of shader
shader.setParameter("texture", sf::Shader::CurrentTexture); //shader.vert
shader.setParameter("opacity", opacity);                    //shader.frag

////////////////////////////

//Delete Text Display
    counter1 = 0;
    for (iter8 = textDisplayArray.begin(); iter8 != textDisplayArray.end(); iter8++)
    {
        if (textDisplayArray[counter1].destroy == true)
        {
            //shader
            opacity -= 0.1;         
            if (opacity <= 0)
            {
                textDisplayArray.erase(iter8);
                opacity = 1;
            }
            shader.setParameter("opacity", opacity);
        }

Answer 1

The RGB value for purple is vec3( 1.0, 0.0, 1.0 ) (maximum red, minimal green and maximum blue). You have to interpolate between your frgment color and the color value of purpel, similar as you do it with opacity . Use mix for this. mix(x, y, a) performs a linear interpolation between x and y using a to weight between them. The return value is computed as x×(1−a)+y×ax×(1−a)+y×a .

uniform sampler2D texture;
uniform float opacity;
uniform float purpleFac;

void main()
{
    vec4 pixel = texture2D(texture, gl_TexCoord[0].xy);
    vec3 mixedCol = mix( vec3( 1.0, 0.0, 1.0 ), pixel.rgb, purpleFac );
    gl_FragColor = vec4( mixedCol , opacity );
}

Note you have to set uniform purpleFac similar as you do it with opacity . purpleFac shoud be in range [0.0, 1.0]. If purpleFac is 1.0 your fragment is colord purple and if it is 0.0 your fragment colol is the color of the texture only.