BitmapFactory.Options中的inDensity和inTargetDensity

Bitmap

影响BitmapFactory decode出的图片的最终大小的不只有inSampleSize属性，还有inDensity和inTargetDensity这两个属性。

• Options中的inDensity属性会根据drawable文件夹的分辨率来赋值。
• Options中的inTartgetDensity会根据屏幕的像素密度来赋值。

在从资源目录加载图片时，这个时候调用的是BitmapFactory#decodeResource方法，内部调用的是decodeResourceStream方法：

#decodeResource

public static Bitmap decodeResource(Resources res, int id, Options opts) {
        validate(opts);
        Bitmap bm = null;
        InputStream is = null; 
        try {
        //openRawResource解析资源，获取相关信息，这里是获取desinity，drawable文件夹下是//0，mdpi===160，hdpi====240 以此类推DisplayMetrics.DENSITY_DEFAULT=160
            final TypedValue value = new TypedValue();
            is = res.openRawResource(id, value);
            bm = decodeResourceStream(res, value, is, null, opts);
        } catch (Exception e) {
            /*  do nothing.
                If the exception happened on open, bm will be null.
                If it happened on close, bm is still valid.
            */
        } finally {
            try {
                if (is != null) is.close();
            } catch (IOException e) {
                // Ignore
            }
        }
        if (bm == null && opts != null && opts.inBitmap != null) {
            throw new IllegalArgumentException("Problem decoding into existing bitmap");
        }
        return bm;
}

#decodeResourceStream

public static Bitmap decodeResourceStream(Resources res, TypedValue value,
            InputStream is, Rect pad, Options opts) {
        if (opts == null) {
            opts = new Options();
        }
        if (opts.inDensity == 0 && value != null) {
            final int density = value.density;
            if (density == TypedValue.DENSITY_DEFAULT) {//图片在drawable目录下
                opts.inDensity = DisplayMetrics.DENSITY_DEFAULT;
            } else if (density != TypedValue.DENSITY_NONE) {//图片在非drawable目录下
                opts.inDensity = density;
            }
        }
        if (opts.inTargetDensity == 0 && res != null) {
            opts.inTargetDensity = res.getDisplayMetrics().densityDpi;
        }
        return decodeStream(is, pad, opts);
}

final TypedValue value = new TypedValue();
is = res.openRawResource(id, value);

在读取资源时，会使用openRawResource方法，这个方法内部会对TypedValue进行赋值，其中包含了原始资源的density等信息，也即是文件夹代表的density；
各文件夹和density的对应关系如下:

drawable ----->   0
ldpi     ----->  120
mdpi     ----->  160
hdpi     ----->  240
xhdpi    ----->  320
xxhdpi   ----->  480
xxxhdpi  ----->  640

drawable文件夹下是0，ldpi==120，mdpi===160，hdpi====240，xhdpi==320以此类推，如果将图片放入默认drawable文件夹(不指定分辨率,即文件夹名后不跟分辨率),则最终会使用默认的Density（DisplayMetrics.DENSITY_DEFAULT=160）。

DisplayMetrics metrics = new DisplayMetrics();
WindowManager wm = (WindowManager) context.getSystemService(Context.WINDOW_SERVICE);
wm.getDefaultDisplay().getMetrics(metrics);
int inTargetDensity = metircs.densityDpi;

屏幕的density可由此代码获得,这个就是inTargetDensity。

根据设备屏幕像素密度，到对应drawable目录去寻找图片，如果能在对应目录找到图片，这时候inTargetDensity/inDensity=1，图片不会做缩放，宽度和高度就是图片原始的像素规格；如果没有在对应的drawable目录找到图片，会到其他规格的drawable目录去找，然后会根据inTargetDensity/inDensity的比例对图片的宽度和高度进行缩放。

输出图片的宽高= 原图片的宽高 / inSampleSize * (inTargetDensity / inDensity)

这个计算仅针对于drawable文件夹的图片来说，而对于一个file或者stream那么inDensity和inTargetDensity是不考虑的!他们默认就是0。

调用decodeResourceStream对原始资源进行解码和适配，实际是原始资源density到设备屏幕density的映射。

注意：inSampleSize只能是2的幂,如不是2的幂下转到最大的2的幂,而且inSampleSize>=1。

接下来，具体看下缩放的实现代码：

Android 4.4之前#decodeStream

public static Bitmap decodeStream(InputStream is, Rect outPadding, Options opts) {
        // we don't throw in this case, thus allowing the caller to only check
        // the cache, and not force the image to be decoded.
        if (is == null) {
            return null;
        }
        // we need mark/reset to work properly
        if (!is.markSupported()) {
            is = new BufferedInputStream(is, DECODE_BUFFER_SIZE);
        }
        // so we can call reset() if a given codec gives up after reading up to
        // this many bytes. FIXME: need to find out from the codecs what this
        // value should be.
        is.mark(1024);
        Bitmap bm;
        boolean finish = true;
        if (is instanceof AssetManager.AssetInputStream) {
            final int asset = ((AssetManager.AssetInputStream) is).getAssetInt();
            if (opts == null || (opts.inScaled && opts.inBitmap == null)) {
                float scale = 1.0f;
                int targetDensity = 0;
                if (opts != null) {
                    final int density = opts.inDensity;
                    targetDensity = opts.inTargetDensity;
                    if (density != 0 && targetDensity != 0) {
                        scale = targetDensity / (float) density;
                    }
                }
                bm = nativeDecodeAsset(asset, outPadding, opts, true, scale);
                if (bm != null && targetDensity != 0) bm.setDensity(targetDensity);
                finish = false;
            } else {
                bm = nativeDecodeAsset(asset, outPadding, opts);
            }
        } else {
            // pass some temp storage down to the native code. 1024 is made up,
            // but should be large enough to avoid too many small calls back
            // into is.read(...) This number is not related to the value passed
            // to mark(...) above.
            byte [] tempStorage = null;
            if (opts != null) tempStorage = opts.inTempStorage;
            if (tempStorage == null) tempStorage = new byte[16 * 1024];
            if (opts == null || (opts.inScaled && opts.inBitmap == null)) {
                float scale = 1.0f;
                int targetDensity = 0;
                if (opts != null) {
                    final int density = opts.inDensity;
                    targetDensity = opts.inTargetDensity;
                    if (density != 0 && targetDensity != 0) {
                        scale = targetDensity / (float) density;
                    }
                }
                bm = nativeDecodeStream(is, tempStorage, outPadding, opts, true, scale);
                if (bm != null && targetDensity != 0) bm.setDensity(targetDensity);
                finish = false;
            } else {
                bm = nativeDecodeStream(is, tempStorage, outPadding, opts);
            }
        }
        if (bm == null && opts != null && opts.inBitmap != null) {
            throw new IllegalArgumentException("Problem decoding into existing bitmap");
        }
        return finish ? finishDecode(bm, outPadding, opts) : bm;
}

#finishDecode

private static Bitmap finishDecode(Bitmap bm, Rect outPadding, Options opts) {
        if (bm == null || opts == null) {
            return bm;
        }
        final int density = opts.inDensity;
        if (density == 0) {
            return bm;
        }
        bm.setDensity(density);
        final int targetDensity = opts.inTargetDensity;
        if (targetDensity == 0 || density == targetDensity || density == opts.inScreenDensity) {
            return bm;
        }
        byte[] np = bm.getNinePatchChunk();
        int[] lb = bm.getLayoutBounds();
        final boolean isNinePatch = np != null && NinePatch.isNinePatchChunk(np);
        if (opts.inScaled || isNinePatch) {
            float scale = targetDensity / (float) density;
            if (scale != 1.0f) {
                final Bitmap oldBitmap = bm;
                bm = Bitmap.createScaledBitmap(oldBitmap, (int) (bm.getWidth() * scale + 0.5f),
                        (int) (bm.getHeight() * scale + 0.5f), true);
                if (bm != oldBitmap) oldBitmap.recycle();
                if (isNinePatch) {
                    np = nativeScaleNinePatch(np, scale, outPadding);
                    bm.setNinePatchChunk(np);
                }
                if (lb != null) {
                    int[] newLb = new int[lb.length];
                    for (int i=0; i<lb.length; i++) {
                        newLb[i] = (int)((lb[i]*scale)+.5f);
                    }
                    bm.setLayoutBounds(newLb);
                }
            }
            bm.setDensity(targetDensity);
        }
        return bm;
}

            ....
            float scale = targetDensity / (float) density;
            if (scale != 1.0f) {
                ....
                final Bitmap oldBitmap = bm;
                bm = Bitmap.createScaledBitmap(oldBitmap, (int) (bm.getWidth() * scale + 0.5f),
                        (int) (bm.getHeight() * scale + 0.5f), true);
                if (bm != oldBitmap) oldBitmap.recycle();
                ....
            }
            ....

在Android 4.4之前，当decodeStream完成后，在设备密度不为0且不等于资源密度时，会执行finishDecode，在finishDecode中会调用createScaledBitmap重新创建bitmap并回收旧的bitmap，也就是说在java层有一个调整（缩放）bitmap的逻辑。

Android 4.4之后#decodeStream

public static Bitmap decodeStream(InputStream is, Rect outPadding, Options opts) {
        // we don't throw in this case, thus allowing the caller to only check
        // the cache, and not force the image to be decoded.
        if (is == null) {
            return null;
        }
        validate(opts);
        Bitmap bm = null;
        Trace.traceBegin(Trace.TRACE_TAG_GRAPHICS, "decodeBitmap");
        try {
            if (is instanceof AssetManager.AssetInputStream) {
                final long asset = ((AssetManager.AssetInputStream) is).getNativeAsset();
                bm = nativeDecodeAsset(asset, outPadding, opts);
            } else {
                bm = decodeStreamInternal(is, outPadding, opts);
            }
            if (bm == null && opts != null && opts.inBitmap != null) {
                throw new IllegalArgumentException("Problem decoding into existing bitmap");
            }
            setDensityFromOptions(bm, opts);
        } finally {
            Trace.traceEnd(Trace.TRACE_TAG_GRAPHICS);
        }
        return bm;
}

#setDensityFromOptions

   private static void setDensityFromOptions(Bitmap outputBitmap, Options opts) {
        if (outputBitmap == null || opts == null) return;
        final int density = opts.inDensity;
        if (density != 0) {
            outputBitmap.setDensity(density);
            final int targetDensity = opts.inTargetDensity;
            if (targetDensity == 0 || density == targetDensity || density == opts.inScreenDensity) {
                return;
            }
            byte[] np = outputBitmap.getNinePatchChunk();
            final boolean isNinePatch = np != null && NinePatch.isNinePatchChunk(np);
            if (opts.inScaled || isNinePatch) {
                outputBitmap.setDensity(targetDensity);
            }
        } else if (opts.inBitmap != null) {
            // bitmap was reused, ensure density is reset
            outputBitmap.setDensity(Bitmap.getDefaultDensity());
        }
}

可以看到，decodeStream方法在执行完成后，会调用setDensityFromOptions方法，该方法主要就是把前面赋值过的两个属性inDensity和inTargetDensity给Bitmap进行赋值，不过并不是直接赋给Bitmap就完了，中间有个判断，当inDensity的值与inTargetDensity或与设备的屏幕Density不相等时，则将应用inTargetDensity的值，如果相等则应用inDensity的值。

在setDensityFromOptions方法中，我们没有看到根据密度调整Bitmap的操作，这是因为Android 4.4以后，为了节省内存，将这个操作放在了native方法中，也就是C层去做Bitmap的缩放。接下来我们看下相关的缩放的代码：

if (env->GetBooleanField(options, gOptions_scaledFieldID)) {
            const int density = env->GetIntField(options, gOptions_densityFieldID);
            const int targetDensity = env->GetIntField(options, gOptions_targetDensityFieldID);
            const int screenDensity = env->GetIntField(options, gOptions_screenDensityFieldID);
            if (density != 0 && targetDensity != 0 && density != screenDensity) {
                scale = (float) targetDensity / density;
            }
        }

更详细的方法代码如下：

#define BYTES_TO_BUFFER 64
static jobject nativeDecodeStream(JNIEnv* env, jobject clazz, jobject is, jbyteArray storage,
    jobject padding, jobject options) {
    jobject bitmap = NULL;
    SkAutoTDelete<SkStream> stream(CreateJavaInputStreamAdaptor(env, is, storage));
    if (stream.get()) {
        SkAutoTDelete<SkStreamRewindable> bufferedStream(
               SkFrontBufferedStream::Create(stream.detach(), BYTES_TO_BUFFER));
        SkASSERT(bufferedStream.get() != NULL);
        bitmap = doDecode(env, bufferedStream, padding, options);
    }
    return bitmap;
}

static jobject doDecode(JNIEnv* env, SkStreamRewindable* stream, jobject padding, jobject options) {
    int sampleSize = 1;
    SkImageDecoder::Mode decodeMode = SkImageDecoder::kDecodePixels_Mode;
    SkColorType prefColorType = kN32_SkColorType;
    bool doDither = true;
    bool isMutable = false;
    float scale = 1.0f;
    bool preferQualityOverSpeed = false;
    bool requireUnpremultiplied = false;
    jobject javaBitmap = NULL;
    if (options != NULL) {
        sampleSize = env->GetIntField(options, gOptions_sampleSizeFieldID);
        if (optionsJustBounds(env, options)) {
            decodeMode = SkImageDecoder::kDecodeBounds_Mode;
        }
        // initialize these, in case we fail later on
        env->SetIntField(options, gOptions_widthFieldID, -1);
        env->SetIntField(options, gOptions_heightFieldID, -1);
        env->SetObjectField(options, gOptions_mimeFieldID, 0);
        jobject jconfig = env->GetObjectField(options, gOptions_configFieldID);
        prefColorType = GraphicsJNI::getNativeBitmapColorType(env, jconfig);
        isMutable = env->GetBooleanField(options, gOptions_mutableFieldID);
        doDither = env->GetBooleanField(options, gOptions_ditherFieldID);
        preferQualityOverSpeed = env->GetBooleanField(options,
                gOptions_preferQualityOverSpeedFieldID);
        requireUnpremultiplied = !env->GetBooleanField(options, gOptions_premultipliedFieldID);
        javaBitmap = env->GetObjectField(options, gOptions_bitmapFieldID);
        if (env->GetBooleanField(options, gOptions_scaledFieldID)) {
            const int density = env->GetIntField(options, gOptions_densityFieldID);
            const int targetDensity = env->GetIntField(options, gOptions_targetDensityFieldID);
            const int screenDensity = env->GetIntField(options, gOptions_screenDensityFieldID);
            if (density != 0 && targetDensity != 0 && density != screenDensity) {
                scale = (float) targetDensity / density;
            }
        }
    }
    const bool willScale = scale != 1.0f;
    SkImageDecoder* decoder = SkImageDecoder::Factory(stream);
    if (decoder == NULL) {
        return nullObjectReturn("SkImageDecoder::Factory returned null");
    }
    decoder->setSampleSize(sampleSize);
    decoder->setDitherImage(doDither);
    decoder->setPreferQualityOverSpeed(preferQualityOverSpeed);
    decoder->setRequireUnpremultipliedColors(requireUnpremultiplied);
    android::Bitmap* reuseBitmap = nullptr;
    unsigned int existingBufferSize = 0;
    if (javaBitmap != NULL) {
        reuseBitmap = GraphicsJNI::getBitmap(env, javaBitmap);
        if (reuseBitmap->peekAtPixelRef()->isImmutable()) {
            ALOGW("Unable to reuse an immutable bitmap as an image decoder target.");
            javaBitmap = NULL;
            reuseBitmap = nullptr;
        } else {
            existingBufferSize = GraphicsJNI::getBitmapAllocationByteCount(env, javaBitmap);
        }
    }
    NinePatchPeeker peeker(decoder);
    decoder->setPeeker(&peeker);
    JavaPixelAllocator javaAllocator(env);
    RecyclingPixelAllocator recyclingAllocator(reuseBitmap, existingBufferSize);
    ScaleCheckingAllocator scaleCheckingAllocator(scale, existingBufferSize);
    SkBitmap::Allocator* outputAllocator = (javaBitmap != NULL) ?
            (SkBitmap::Allocator*)&recyclingAllocator : (SkBitmap::Allocator*)&javaAllocator;
    if (decodeMode != SkImageDecoder::kDecodeBounds_Mode) {
        if (!willScale) {
            // If the java allocator is being used to allocate the pixel memory, the decoder
            // need not write zeroes, since the memory is initialized to 0.
            decoder->setSkipWritingZeroes(outputAllocator == &javaAllocator);
            decoder->setAllocator(outputAllocator);
        } else if (javaBitmap != NULL) {
            // check for eventual scaled bounds at allocation time, so we don't decode the bitmap
            // only to find the scaled result too large to fit in the allocation
            decoder->setAllocator(&scaleCheckingAllocator);
        }
    }
    // Only setup the decoder to be deleted after its stack-based, refcounted
    // components (allocators, peekers, etc) are declared. This prevents RefCnt
    // asserts from firing due to the order objects are deleted from the stack.
    SkAutoTDelete<SkImageDecoder> add(decoder);
    AutoDecoderCancel adc(options, decoder);
    // To fix the race condition in case "requestCancelDecode"
    // happens earlier than AutoDecoderCancel object is added
    // to the gAutoDecoderCancelMutex linked list.
    if (options != NULL && env->GetBooleanField(options, gOptions_mCancelID)) {
        return nullObjectReturn("gOptions_mCancelID");
    }
    SkBitmap decodingBitmap;
    if (decoder->decode(stream, &decodingBitmap, prefColorType, decodeMode)
                != SkImageDecoder::kSuccess) {
        return nullObjectReturn("decoder->decode returned false");
    }
    int scaledWidth = decodingBitmap.width();
    int scaledHeight = decodingBitmap.height();
    if (willScale && decodeMode != SkImageDecoder::kDecodeBounds_Mode) {
        scaledWidth = int(scaledWidth * scale + 0.5f);
        scaledHeight = int(scaledHeight * scale + 0.5f);
    }
    // update options (if any)
    if (options != NULL) {
        jstring mimeType = getMimeTypeString(env, decoder->getFormat());
        if (env->ExceptionCheck()) {
            return nullObjectReturn("OOM in getMimeTypeString()");
        }
        env->SetIntField(options, gOptions_widthFieldID, scaledWidth);
        env->SetIntField(options, gOptions_heightFieldID, scaledHeight);
        env->SetObjectField(options, gOptions_mimeFieldID, mimeType);
    }
    // if we're in justBounds mode, return now (skip the java bitmap)
    if (decodeMode == SkImageDecoder::kDecodeBounds_Mode) {
        return NULL;
    }
    jbyteArray ninePatchChunk = NULL;
    if (peeker.mPatch != NULL) {
        if (willScale) {
            scaleNinePatchChunk(peeker.mPatch, scale, scaledWidth, scaledHeight);
        }
        size_t ninePatchArraySize = peeker.mPatch->serializedSize();
        ninePatchChunk = env->NewByteArray(ninePatchArraySize);
        if (ninePatchChunk == NULL) {
            return nullObjectReturn("ninePatchChunk == null");
        }
        jbyte* array = (jbyte*) env->GetPrimitiveArrayCritical(ninePatchChunk, NULL);
        if (array == NULL) {
            return nullObjectReturn("primitive array == null");
        }
        memcpy(array, peeker.mPatch, peeker.mPatchSize);
        env->ReleasePrimitiveArrayCritical(ninePatchChunk, array, 0);
    }
    jobject ninePatchInsets = NULL;
    if (peeker.mHasInsets) {
        ninePatchInsets = env->NewObject(gInsetStruct_class, gInsetStruct_constructorMethodID,
                peeker.mOpticalInsets[0], peeker.mOpticalInsets[1], peeker.mOpticalInsets[2], peeker.mOpticalInsets[3],
                peeker.mOutlineInsets[0], peeker.mOutlineInsets[1], peeker.mOutlineInsets[2], peeker.mOutlineInsets[3],
                peeker.mOutlineRadius, peeker.mOutlineAlpha, scale);
        if (ninePatchInsets == NULL) {
            return nullObjectReturn("nine patch insets == null");
        }
        if (javaBitmap != NULL) {
            env->SetObjectField(javaBitmap, gBitmap_ninePatchInsetsFieldID, ninePatchInsets);
        }
    }
    SkBitmap outputBitmap;
    if (willScale) {
        // This is weird so let me explain: we could use the scale parameter
        // directly, but for historical reasons this is how the corresponding
        // Dalvik code has always behaved. We simply recreate the behavior here.
        // The result is slightly different from simply using scale because of
        // the 0.5f rounding bias applied when computing the target image size
        const float sx = scaledWidth / float(decodingBitmap.width());
        const float sy = scaledHeight / float(decodingBitmap.height());
        // TODO: avoid copying when scaled size equals decodingBitmap size
        SkColorType colorType = colorTypeForScaledOutput(decodingBitmap.colorType());
        // FIXME: If the alphaType is kUnpremul and the image has alpha, the
        // colors may not be correct, since Skia does not yet support drawing
        // to/from unpremultiplied bitmaps.
        outputBitmap.setInfo(SkImageInfo::Make(scaledWidth, scaledHeight,
                colorType, decodingBitmap.alphaType()));
        if (!outputBitmap.tryAllocPixels(outputAllocator, NULL)) {
            return nullObjectReturn("allocation failed for scaled bitmap");
        }
        // If outputBitmap's pixels are newly allocated by Java, there is no need
        // to erase to 0, since the pixels were initialized to 0.
        if (outputAllocator != &javaAllocator) {
            outputBitmap.eraseColor(0);
        }
        SkPaint paint;
        paint.setFilterQuality(kLow_SkFilterQuality);
        SkCanvas canvas(outputBitmap);
        canvas.scale(sx, sy);
        canvas.drawARGB(0x00, 0x00, 0x00, 0x00);
        canvas.drawBitmap(decodingBitmap, 0.0f, 0.0f, &paint);
    } else {
        outputBitmap.swap(decodingBitmap);
    }
    if (padding) {
        if (peeker.mPatch != NULL) {
            GraphicsJNI::set_jrect(env, padding,
                    peeker.mPatch->paddingLeft, peeker.mPatch->paddingTop,
                    peeker.mPatch->paddingRight, peeker.mPatch->paddingBottom);
        } else {
            GraphicsJNI::set_jrect(env, padding, -1, -1, -1, -1);
        }
    }
    // if we get here, we're in kDecodePixels_Mode and will therefore
    // already have a pixelref installed.
    if (outputBitmap.pixelRef() == NULL) {
        return nullObjectReturn("Got null SkPixelRef");
    }
    if (!isMutable && javaBitmap == NULL) {
        // promise we will never change our pixels (great for sharing and pictures)
        outputBitmap.setImmutable();
    }
    if (javaBitmap != NULL) {
        bool isPremultiplied = !requireUnpremultiplied;
        GraphicsJNI::reinitBitmap(env, javaBitmap, outputBitmap.info(), isPremultiplied);
        outputBitmap.notifyPixelsChanged();
        // If a java bitmap was passed in for reuse, pass it back
        return javaBitmap;
    }
    int bitmapCreateFlags = 0x0;
    if (isMutable) bitmapCreateFlags |= GraphicsJNI::kBitmapCreateFlag_Mutable;
    if (!requireUnpremultiplied) bitmapCreateFlags |= GraphicsJNI::kBitmapCreateFlag_Premultiplied;
    // now create the java bitmap
    return GraphicsJNI::createBitmap(env, javaAllocator.getStorageObjAndReset(),
            bitmapCreateFlags, ninePatchChunk, ninePatchInsets, -1);
}

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