In Metal Shader, how to treat a specific color in texture2d as transparent, for example, white？

In Metal Shader, you can replace the color you want to make transparent by comparing it with a literal color using the texture2d and its sampling coordinates passed to the fragment shader.

Here is a snippet of an example code. Assuming your texture code is as follows:

fragment float4 myFragmentShader(VertexOut vert [[stage_in]],
                                 texture2d<float, access::sample> texture [[texture(0)]]) {
    constexpr sampler texSampler(mag_filter::linear, min_filter::linear);
    float4 color = texture.sample(texSampler, vert.texCoord);
    return color;
}

Then, if you want to make the white color transparent, you can add the following code:

fragment float4 myFragmentShader(VertexOut vert [[stage_in]],
                                 texture2d<float, access::sample> texture [[texture(0)]]) {
    constexpr sampler texSampler(mag_filter::linear, min_filter::linear);
    float4 color = texture.sample(texSampler, vert.texCoord);

    // Process white as transparent, you can adjust the following condition and literal color arbitrarily
    if (color.r > 0.99 && color.g > 0.99 && color.b > 0.99)  {
        return float4(0, 0, 0, 0);
    }

    return color;
}

In the above code, we added some logic to detect if the color is close to pure white (i.e., all three channels are greater than 0.99), and if the condition is met, we replace it with a transparent color. You can change the conditions and literal colors arbitrarily to achieve the desired effect.

Note: In some cases, comparing the equality of floating point numbers to detect colors may encounter precision issues, so some optimizations may be needed in actual use.