c# - 是否存在使白色不可见的 alpha 值？

Question

如果我使用以下内容

for(int i = 255; i > 0; i--)
{
    Color transparentBlack = new Color(0, 0, 0, i);
}

我有使用这种颜色绘制的对象的效果，从黑色变为浅灰色，然后当 alpha 值变为零时不可见。但是，如果我从白色值开始:

new Color(255, 255, 255, i);

对象永远不会变得不可见，只会保持白色。我还注意到，如果我使用比黑色稍浅的值(比如 50、50、50)，绘图会从深色变为不可见，再变为白色。

我假设我只是不明白 alpha 混合是如何工作的，但是有没有办法让白色逐渐变成半透明？

编辑:我绘制的背景是 Color.CornflowerBlue (100,149,237,255)

编辑:再现解释的示例 XNA 程序。使用；创建一个新的 XNA Game Studio 4.0 项目 - Windows Game (4.0)，将其命名为 AlphaBlendTest - & 在内容项目中添加一个新的 SpriteFont 并将其命名为 testfont

using System;
using System.Collections.Generic;
using System.Linq;
using Microsoft.Xna.Framework;
using Microsoft.Xna.Framework.Audio;
using Microsoft.Xna.Framework.Content;
using Microsoft.Xna.Framework.GamerServices;
using Microsoft.Xna.Framework.Graphics;
using Microsoft.Xna.Framework.Input;
using Microsoft.Xna.Framework.Media;

namespace AlphaBlendTest
{
    /// <summary>
    /// This is the main type for your game
    /// </summary>
    public class Game1 : Microsoft.Xna.Framework.Game
    {
        GraphicsDeviceManager graphics;
        SpriteBatch spriteBatch;


    SpriteFont font;

    public Game1()
    {
        graphics = new GraphicsDeviceManager(this);
        Content.RootDirectory = "Content";
    }

    /// <summary>
    /// Allows the game to perform any initialization it needs to before starting to run.
    /// This is where it can query for any required services and load any non-graphic
    /// related content.  Calling base.Initialize will enumerate through any components
    /// and initialize them as well.
    /// </summary>
    protected override void Initialize()
    {
        // TODO: Add your initialization logic here

        base.Initialize();
    }

    /// <summary>
    /// LoadContent will be called once per game and is the place to load
    /// all of your content.
    /// </summary>
    protected override void LoadContent()
    {
        // Create a new SpriteBatch, which can be used to draw textures.
        spriteBatch = new SpriteBatch(GraphicsDevice);

        font = Content.Load<SpriteFont>("testfont");
    }

    /// <summary>
    /// UnloadContent will be called once per game and is the place to unload
    /// all content.
    /// </summary>
    protected override void UnloadContent()
    {
        // TODO: Unload any non ContentManager content here
    }

    /// <summary>
    /// Allows the game to run logic such as updating the world,
    /// checking for collisions, gathering input, and playing audio.
    /// </summary>
    /// <param name="gameTime">Provides a snapshot of timing values.</param>
    protected override void Update(GameTime gameTime)
    {
        // Allows the game to exit
        if (GamePad.GetState(PlayerIndex.One).Buttons.Back == ButtonState.Pressed)
            this.Exit();

        // TODO: Add your update logic here

        base.Update(gameTime);
    }

    /// <summary>
    /// This is called when the game should draw itself.
    /// </summary>
    /// <param name="gameTime">Provides a snapshot of timing values.</param>
    protected override void Draw(GameTime gameTime)
    {
        GraphicsDevice.Clear(Color.CornflowerBlue);
        spriteBatch.Begin();

        //spriteBatch.Draw(tR, new Rectangle(100, 100, 100, 100), Color.Red);

        Vector2 v2 = new Vector2(0, 0);
        spriteBatch.DrawString(font, "Test - White", v2, Color.White);

        v2.Y = v2.Y + 50;

        spriteBatch.DrawString(font, "Test - Black", v2, Color.Black);

        v2.Y = v2.Y + 50;

        Color BlackTransparent = new Color(0, 0, 0, 0);
        spriteBatch.DrawString(font, "Test - Black Transparent", v2, BlackTransparent);

        v2.Y = v2.Y + 50;
        Color WhiteTransparent = new Color(255, 255, 255, 0);
        spriteBatch.DrawString(font, "Test - White Transparent", v2, WhiteTransparent);

        spriteBatch.End();

        base.Draw(gameTime);
    }
}
}

编辑:这是这段代码绘制的图像:

编辑:其中一条评论担心这是一个仅与文本相关的窗口“功能”。我使用文本作为例子来保持演示程序的小；使用图像进行测试会得到相同的结果。

将方框添加到演示程序中；创建一个方形白色 PNG 图像并将其添加到内容目录(使用内容管道的默认值)。然后将其添加到类中:

Texture2D tWhiteBox;

将此添加到加载方法中:

tWhiteBox = Content.Load<Texture2D>("whitebox");

然后在 draw 中的其他 draw 语句下面添加以下内容:

v2.Y = v2.Y + 50;
spriteBatch.Draw(tWhiteBox, v2, WhiteTransparent);

Answer 1

灰色是您在 XNA 4.0 中得到的(如果您尝试只调整 alpha channel 而不同时调整 RGB channel ，则为全白色)。透明度是使用预乘 alpha 完成的(默认情况下)。如果您正在寻找旧的非 pre-mul XNA 3.1 行为，请参阅 Shawn Hargreaves 的这篇文章:http://blogs.msdn.com/b/shawnhar/archive/2010/04/08/premultiplied-alpha-in-xna-game-studio-4-0.aspx .在帖子的底部，它会告诉您如何操作。

我建议在阅读之前先阅读他关于预乘 alpha 的所有帖子(您可以使用他页面顶部附近左侧的“博客索引”链接)- pre-mul 确实很多更好，逻辑上更一致。