Downsampler - 采样链路
采样流程：
- 从原图尺寸和目标尺寸算出理想缩放比
- 用
highestOneBit变成 2 的幂 →inSampleSize - 模拟 BitmapFactory 各格式的舍入规则，精确预测输出尺寸
- 用密度缩放（与普通缩放不同）做第二级精调，弥补 2 的幂不够精确的缺陷
- 把参数写进
BitmapFactory.Options，交给系统解码
private static void calculateScaling(
ImageType imageType,
ImageReader imageReader,
DecodeCallbacks decodeCallbacks,
BitmapPool bitmapPool,
DownsampleStrategy downsampleStrategy,
int degreesToRotate,
int sourceWidth,
int sourceHeight,
int targetWidth,
int targetHeight,
BitmapFactory.Options options)
throws IOException {
// We can't downsample source content if we can't determine its dimensions.
if (sourceWidth <= 0 || sourceHeight <= 0) {
if (Log.isLoggable(TAG, Log.DEBUG)) {
Log.d(
TAG,
"Unable to determine dimensions for: "
+ imageType
+ " with target ["
+ targetWidth
+ "x"
+ targetHeight
+ "]");
}
return;
}
// 处理旋转
int orientedSourceWidth = sourceWidth;
int orientedSourceHeight = sourceHeight;
// If we're rotating the image +-90 degrees, we need to downsample accordingly so the image
// width is decreased to near our target's height and the image height is decreased to near
// our target width.
//noinspection SuspiciousNameCombination
if (isRotationRequired(degreesToRotate)) {
orientedSourceWidth = sourceHeight;
orientedSourceHeight = sourceWidth;
}
// 计算精确缩放因子
final float exactScaleFactor =
downsampleStrategy.getScaleFactor(
orientedSourceWidth, orientedSourceHeight, targetWidth, targetHeight);
if (exactScaleFactor <= 0f) {
throw new IllegalArgumentException(
"Cannot scale with factor: "
+ exactScaleFactor
+ " from: "
+ downsampleStrategy
+ ", source: ["
+ sourceWidth
+ "x"
+ sourceHeight
+ "]"
+ ", target: ["
+ targetWidth
+ "x"
+ targetHeight
+ "]");
}
SampleSizeRounding rounding =
downsampleStrategy.getSampleSizeRounding(
orientedSourceWidth, orientedSourceHeight, targetWidth, targetHeight);
if (rounding == null) {
throw new IllegalArgumentException("Cannot round with null rounding");
}
// 计算"理想输出尺寸"
int outWidth = round(exactScaleFactor * orientedSourceWidth);
int outHeight = round(exactScaleFactor * orientedSourceHeight);
// 算出"实际需要的倍数"
int widthScaleFactor = orientedSourceWidth / outWidth;
int heightScaleFactor = orientedSourceHeight / outHeight;
// TODO: This isn't really right for both CenterOutside and CenterInside. Consider allowing
// DownsampleStrategy to pick, or trying to do something more sophisticated like picking the
// scale factor that leads to an exact match.
int scaleFactor =
rounding == SampleSizeRounding.MEMORY
? Math.max(widthScaleFactor, heightScaleFactor)
: Math.min(widthScaleFactor, heightScaleFactor);
// 转换成 2 的幂 —— 第一次"砍"
int powerOfTwoSampleSize;
// BitmapFactory does not support downsampling wbmp files on platforms <= M. See b/27305903.
if (Build.VERSION.SDK_INT <= 23
&& NO_DOWNSAMPLE_PRE_N_MIME_TYPES.contains(options.outMimeType)) {
// WBMP/ICO，且 Android M 及以下
powerOfTwoSampleSize = 1;
} else {
// scaleFactor 在二进制下的最高位
powerOfTwoSampleSize = Math.max(1, Integer.highestOneBit(scaleFactor));
if (rounding == SampleSizeRounding.MEMORY
&& powerOfTwoSampleSize < (1.f / exactScaleFactor)) {
// 还可以再左移一位
powerOfTwoSampleSize = powerOfTwoSampleSize << 1;
}
}
// Here we mimic framework logic for determining how inSampleSize division is rounded on various
// versions of Android. The logic here has been tested on emulators for Android versions 15-26.
// PNG - Always uses floor
// JPEG - Always uses ceiling
// Webp - Prior to N, always uses floor. At and after N, always uses round.
options.inSampleSize = powerOfTwoSampleSize;
int powerOfTwoWidth;
int powerOfTwoHeight;
if (imageType == ImageType.JPEG) {
// libjpegturbo can downsample up to a sample size of 8. libjpegturbo uses ceiling to round.
// After libjpegturbo's native rounding, skia does a secondary scale using floor
// (integer division). Here we replicate that logic.
// libjpegturbo 单次最多采样 8 倍，用 ceil
int nativeScaling = Math.min(powerOfTwoSampleSize, 8);
powerOfTwoWidth = (int) Math.ceil(orientedSourceWidth / (float) nativeScaling);
powerOfTwoHeight = (int) Math.ceil(orientedSourceHeight / (float) nativeScaling);
// 超过 8 的部分
int secondaryScaling = powerOfTwoSampleSize / 8;
if (secondaryScaling > 0) {
powerOfTwoWidth = powerOfTwoWidth / secondaryScaling;
powerOfTwoHeight = powerOfTwoHeight / secondaryScaling;
}
} else if (imageType == ImageType.PNG || imageType == ImageType.PNG_A) {
powerOfTwoWidth = (int) Math.floor(orientedSourceWidth / (float) powerOfTwoSampleSize);
powerOfTwoHeight = (int) Math.floor(orientedSourceHeight / (float) powerOfTwoSampleSize);
} else if (imageType.isWebp()) {
if (Build.VERSION.SDK_INT >= Build.VERSION_CODES.N) {
powerOfTwoWidth = Math.round(orientedSourceWidth / (float) powerOfTwoSampleSize);
powerOfTwoHeight = Math.round(orientedSourceHeight / (float) powerOfTwoSampleSize);
} else {
powerOfTwoWidth = (int) Math.floor(orientedSourceWidth / (float) powerOfTwoSampleSize);
powerOfTwoHeight = (int) Math.floor(orientedSourceHeight / (float) powerOfTwoSampleSize);
}
} else if (orientedSourceWidth % powerOfTwoSampleSize != 0
|| orientedSourceHeight % powerOfTwoSampleSize != 0) {
// If we're not confident the image is in one of our types, fall back to checking the
// dimensions again. inJustDecodeBounds decodes do obey inSampleSize.
int[] dimensions = getDimensions(imageReader, options, decodeCallbacks, bitmapPool);
// Power of two downsampling in BitmapFactory uses a variety of random factors to determine
// rounding that we can't reliably replicate for all image formats. Use ceiling here to make
// sure that we at least provide a Bitmap that's large enough to fit the content we're going
// to load.
powerOfTwoWidth = dimensions[0];
powerOfTwoHeight = dimensions[1];
} else {
powerOfTwoWidth = orientedSourceWidth / powerOfTwoSampleSize;
powerOfTwoHeight = orientedSourceHeight / powerOfTwoSampleSize;
}
// 计算二次缩放的密度比
double adjustedScaleFactor =
downsampleStrategy.getScaleFactor(
powerOfTwoWidth, powerOfTwoHeight, targetWidth, targetHeight);
// Density scaling is only supported if inBitmap is null prior to KitKat. Avoid setting
// densities here so we calculate the final Bitmap size correctly.
// 设置密度字段（Android 4.4+）
if (Build.VERSION.SDK_INT >= Build.VERSION_CODES.KITKAT) {
options.inTargetDensity = adjustTargetDensityForError(adjustedScaleFactor);
options.inDensity = getDensityMultiplier(adjustedScaleFactor);
}
if (isScaling(options)) {
options.inScaled = true;
} else {
options.inDensity = options.inTargetDensity = 0;
}
if (Log.isLoggable(TAG, Log.VERBOSE)) {
Log.v(
TAG,
"Calculate scaling"
+ ", source: ["
+ sourceWidth
+ "x"
+ sourceHeight
+ "]"
+ ", degreesToRotate: "
+ degreesToRotate
+ ", target: ["
+ targetWidth
+ "x"
+ targetHeight
+ "]"
+ ", power of two scaled: ["
+ powerOfTwoWidth
+ "x"
+ powerOfTwoHeight
+ "]"
+ ", exact scale factor: "
+ exactScaleFactor
+ ", power of 2 sample size: "
+ powerOfTwoSampleSize
+ ", adjusted scale factor: "
+ adjustedScaleFactor
+ ", target density: "
+ options.inTargetDensity
+ ", density: "
+ options.inDensity);
}
}