Android像素编辑器极端宽高比下Bitmap像素矩形化问题

我正在开发一款Android像素艺术编辑器，发现创建宽高比极端的Bitmap（例如3x300、5x90或反向比例）时，像素会呈现轻微矩形。但宽高比相近的Bitmap（如50x40、90x80）不会出现此问题。

相关代码 #

边界Rect计算代码 #

private fun setBoundingRect() {
    val ratio = if (bitmapWidth > bitmapHeight) {
        bitmapHeight.toDouble() / bitmapWidth.toDouble()
    } else {
        bitmapWidth.toDouble() / bitmapHeight.toDouble()
    }

    val rectW: Int = if (bitmapWidth > bitmapHeight) {
        width
    } else if (bitmapHeight > bitmapWidth) {
        (height * ratio).toInt()
    } else {
        width
    }

    val rectH: Int = if (bitmapWidth > bitmapHeight)  {
        (width * ratio).toInt()
    } else if (bitmapHeight > bitmapWidth) {
        height
    } else {
        width
    }

    val canvasCenter = Point(width / 2, height / 2)

    val left = canvasCenter.x - rectW / 2
    val top = canvasCenter.y - rectH / 2
    val right = canvasCenter.x + rectW / 2
    val bottom = canvasCenter.y + rectH / 2

    boundingRect = Rect(left, top, right, bottom)
}

逻辑说明：通过计算宽高比作为缩放因子，对容器尺寸进行缩放以匹配Bitmap比例，多数情况有效，但极端比例时出问题。

onDraw方法代码 #

override fun onDraw(canvas: Canvas) {
    if (::drawingViewBitmap.isInitialized) {
        canvas.drawRect(boundingRect, PaintData.rectPaint)
        canvas.drawBitmap(drawingViewBitmap, null, boundingRect, null)
        drawGrid(canvas)
    }
}

问题分析与解决方案 #

问题核心在于整数取整导致的比例偏差：计算rectW和rectH时直接用toInt()截断小数，极端宽高比下，这种截断会让boundingRect的宽高比与原始Bitmap产生细微差异，最终导致像素拉伸变形。另外，默认的Bitmap缩放滤镜也会加剧这个问题。

修复步骤： #

1. 修正宽高比计算，消除比例偏差
   不要单独计算宽和高，应基于容器尺寸和Bitmap原始比例算出统一缩放因子，再同步计算宽高，确保boundingRect比例与Bitmap完全一致：

   private fun setBoundingRect() {
       val bitmapRatio = bitmapWidth.toDouble() / bitmapHeight.toDouble()
       val containerRatio = width.toDouble() / height.toDouble()
   
       val scaleFactor: Double = if (bitmapRatio > containerRatio) {
           // Bitmap更宽，以容器宽度为基准缩放
           width.toDouble() / bitmapWidth.toDouble()
       } else {
           // Bitmap更高，以容器高度为基准缩放
           height.toDouble() / bitmapHeight.toDouble()
       }
   
       val rectW = (bitmapWidth * scaleFactor).toInt()
       val rectH = (bitmapHeight * scaleFactor).toInt()
   
       val canvasCenter = Point(width / 2, height / 2)
       val left = canvasCenter.x - rectW / 2
       val top = canvasCenter.y - rectH / 2
       val right = canvasCenter.x + rectW / 2
       val bottom = canvasCenter.y + rectH / 2
   
       boundingRect = Rect(left, top, right, bottom)
   }
   

2. 禁用缩放滤镜，保持像素锐利
   绘制Bitmap时使用关闭过滤的Paint，避免双线性过滤导致的像素模糊与拉伸：

   // 提前初始化绘制Bitmap的Paint
   private val bitmapPaint = Paint().apply {
       isFilterBitmap = false
       isAntiAlias = false
   }
   
   override fun onDraw(canvas: Canvas) {
       if (::drawingViewBitmap.isInitialized) {
           canvas.drawRect(boundingRect, PaintData.rectPaint)
           canvas.drawBitmap(drawingViewBitmap, null, boundingRect, bitmapPaint)
           drawGrid(canvas)
       }
   }
   

3. 额外校验：匹配Bitmap密度
   确保drawingViewBitmap的密度与当前View的屏幕密度一致，避免系统自动密度缩放导致变形：

   Bitmap.createBitmap(bitmapWidth, bitmapHeight, Bitmap.Config.ARGB_8888).apply {
       density = resources.displayMetrics.densityDpi
   }
   

内容的提问来源于stack exchange，提问作者thebluepandabear