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网页游戏中的文本渲染 ​

文本无处不在——分数、对话、菜单、伤害数字。这篇教程涵盖不同方案以及何时用哪种。

1）Canvas 文本基础 ​

最简单的方式：

js
ctx.font = '24px Arial'
ctx.fillStyle = '#ffffff'
ctx.textAlign = 'center'
ctx.textBaseline = 'middle'
ctx.fillText('Score: 1000', canvas.width / 2, 50)

2）带描边的文本 ​

js
function drawTextWithOutline(ctx, text, x, y, fillColor, outlineColor, outlineWidth = 2) {
  ctx.font = '24px Arial'
  ctx.textAlign = 'center'
  ctx.textBaseline = 'middle'
  
  ctx.strokeStyle = outlineColor
  ctx.lineWidth = outlineWidth
  ctx.strokeText(text, x, y)
  
  ctx.fillStyle = fillColor
  ctx.fillText(text, x, y)
}

drawTextWithOutline(ctx, 'GAME OVER', 400, 300, '#fff', '#000', 3)

3）带阴影的文本 ​

js
function drawTextWithShadow(ctx, text, x, y, color, shadowOffset = 2) {
  ctx.font = '32px sans-serif'
  ctx.textAlign = 'left'
  ctx.textBaseline = 'top'
  
  // 阴影
  ctx.fillStyle = 'rgba(0, 0, 0, 0.5)'
  ctx.fillText(text, x + shadowOffset, y + shadowOffset)
  
  // 主文本
  ctx.fillStyle = color
  ctx.fillText(text, x, y)
}

4）位图字体渲染 ​

适合像素风游戏或希望跨平台外观一致的场景：

js
class BitmapFont {
  constructor(image, charWidth, charHeight, chars) {
    this.image = image
    this.charWidth = charWidth
    this.charHeight = charHeight
    this.chars = chars
  }
  
  draw(ctx, text, x, y, scale = 1) {
    const w = this.charWidth * scale
    const h = this.charHeight * scale
    
    for (let i = 0; i < text.length; i++) {
      const charIndex = this.chars.indexOf(text[i].toUpperCase())
      if (charIndex === -1) continue
      
      const sx = (charIndex % 16) * this.charWidth
      const sy = Math.floor(charIndex / 16) * this.charHeight
      
      ctx.drawImage(
        this.image,
        sx, sy, this.charWidth, this.charHeight,
        Math.round(x + i * w), Math.round(y), w, h
      )
    }
  }
  
  measureText(text, scale = 1) {
    return text.length * this.charWidth * scale
  }
}

// 用法
const font = new BitmapFont(fontImage, 8, 8, 
  'ABCDEFGHIJKLMNOPQRSTUVWXYZ0123456789.,!?-+:; '
)
font.draw(ctx, 'SCORE: 1000', 10, 10, 2)

5）预渲染文本以提升性能 ​

对静态文本，渲染一次反复使用：

js
function createTextImage(text, font, color) {
  const tempCanvas = document.createElement('canvas')
  const tempCtx = tempCanvas.getContext('2d')
  
  tempCtx.font = font
  const metrics = tempCtx.measureText(text)
  
  tempCanvas.width = Math.ceil(metrics.width)
  tempCanvas.height = parseInt(font) * 1.5
  
  tempCtx.font = font
  tempCtx.fillStyle = color
  tempCtx.textBaseline = 'top'
  tempCtx.fillText(text, 0, 0)
  
  return tempCanvas
}

// 只创建一次
const titleImage = createTextImage('MY GAME', '48px Impact', '#fff')

// 多次绘制（快）
ctx.drawImage(titleImage, x, y)

6）打字机效果 ​

js
class TypewriterText {
  constructor(text, x, y, charsPerSecond = 20) {
    this.fullText = text
    this.x = x
    this.y = y
    this.speed = 1 / charsPerSecond
    this.timer = 0
    this.charIndex = 0
  }
  
  update(dt) {
    this.timer += dt
    while (this.timer >= this.speed && this.charIndex < this.fullText.length) {
      this.charIndex++
      this.timer -= this.speed
    }
  }
  
  draw(ctx) {
    ctx.font = '18px monospace'
    ctx.fillStyle = '#fff'
    ctx.fillText(this.fullText.slice(0, this.charIndex), this.x, this.y)
  }
  
  isComplete() {
    return this.charIndex >= this.fullText.length
  }
  
  skip() {
    this.charIndex = this.fullText.length
  }
}

7）自动换行文本 ​

js
function wrapText(ctx, text, maxWidth) {
  const words = text.split(' ')
  const lines = []
  let currentLine = ''
  
  for (const word of words) {
    const testLine = currentLine ? currentLine + ' ' + word : word
    const metrics = ctx.measureText(testLine)
    
    if (metrics.width > maxWidth && currentLine) {
      lines.push(currentLine)
      currentLine = word
    } else {
      currentLine = testLine
    }
  }
  
  if (currentLine) lines.push(currentLine)
  return lines
}

function drawWrappedText(ctx, text, x, y, maxWidth, lineHeight) {
  const lines = wrapText(ctx, text, maxWidth)
  for (let i = 0; i < lines.length; i++) {
    ctx.fillText(lines[i], x, y + i * lineHeight)
  }
}

8）对话框系统 ​

js
class DialogueBox {
  constructor(x, y, width, height) {
    this.x = x
    this.y = y
    this.width = width
    this.height = height
    this.padding = 16
    this.lineHeight = 24
    this.text = ''
    this.displayedChars = 0
    this.charsPerSecond = 30
    this.timer = 0
  }
  
  show(text) {
    this.text = text
    this.displayedChars = 0
    this.timer = 0
  }
  
  update(dt) {
    if (this.displayedChars < this.text.length) {
      this.timer += dt
      const charsToAdd = Math.floor(this.timer * this.charsPerSecond)
      this.displayedChars = Math.min(this.displayedChars + charsToAdd, this.text.length)
      this.timer = this.timer % (1 / this.charsPerSecond)
    }
  }
  
  skip() {
    this.displayedChars = this.text.length
  }
  
  isComplete() {
    return this.displayedChars >= this.text.length
  }
  
  draw(ctx) {
    // 背景框
    ctx.fillStyle = 'rgba(0, 0, 0, 0.8)'
    ctx.fillRect(this.x, this.y, this.width, this.height)
    
    // 边框
    ctx.strokeStyle = '#fff'
    ctx.lineWidth = 2
    ctx.strokeRect(this.x, this.y, this.width, this.height)
    
    // 文本
    ctx.font = '18px sans-serif'
    ctx.fillStyle = '#fff'
    ctx.textBaseline = 'top'
    
    const displayText = this.text.slice(0, this.displayedChars)
    const maxWidth = this.width - this.padding * 2
    const lines = wrapText(ctx, displayText, maxWidth)
    
    for (let i = 0; i < lines.length; i++) {
      ctx.fillText(
        lines[i],
        this.x + this.padding,
        this.y + this.padding + i * this.lineHeight
      )
    }
    
    // 继续指示
    if (this.isComplete()) {
      ctx.fillText('▼', this.x + this.width - 24, this.y + this.height - 24)
    }
  }
}

9）浮动伤害数字 ​

js
class FloatingText {
  constructor(text, x, y, color = '#fff') {
    this.text = text
    this.x = x
    this.y = y
    this.color = color
    this.vy = -60
    this.lifetime = 1
    this.age = 0
  }
  
  update(dt) {
    this.y += this.vy * dt
    this.vy *= 0.95
    this.age += dt
  }
  
  draw(ctx) {
    const alpha = 1 - (this.age / this.lifetime)
    ctx.font = 'bold 20px sans-serif'
    ctx.textAlign = 'center'
    ctx.fillStyle = this.color
    ctx.globalAlpha = alpha
    ctx.fillText(this.text, Math.round(this.x), Math.round(this.y))
    ctx.globalAlpha = 1
  }
  
  isExpired() {
    return this.age >= this.lifetime
  }
}

// 管理器
const floatingTexts = []

function spawnDamageNumber(damage, x, y) {
  const color = damage > 50 ? '#ff4444' : '#ffff44'
  floatingTexts.push(new FloatingText(damage.toString(), x, y, color))
}

function updateFloatingTexts(dt) {
  for (let i = floatingTexts.length - 1; i >= 0; i--) {
    floatingTexts[i].update(dt)
    if (floatingTexts[i].isExpired()) {
      floatingTexts.splice(i, 1)
    }
  }
}

10）WebGL 文本（SDF 字体） ​

对 WebGL 游戏，Signed Distance Field 字体能干净地缩放：

js
// SDF 片元 shader
const sdfFragmentShader = `
  precision mediump float;
  
  uniform sampler2D u_texture;
  uniform vec4 u_color;
  varying vec2 v_texCoord;
  
  void main() {
    float distance = texture2D(u_texture, v_texCoord).a;
    float alpha = smoothstep(0.4, 0.6, distance);
    gl_FragColor = vec4(u_color.rgb, u_color.a * alpha);
  }
`

// 像 msdf-bmfont-xml 这样的库可以生成 SDF 字体图集

对大多数 2D Canvas 游戏，位图字体或预渲染 Canvas 文本就够用了。

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