Android Canvas drawBitmap 尽量使用Matrix缩放_android drawbitmap matr

测试代码

private void testDrawBitmap() {
    Bitmap srcBitmap = BitmapFactory.decodeResource(getResources(), R.mipmap.ic_launcher);

    int dstWidth = 192 * 9, dstHeight = 192 * 9;
    Bitmap desBitmap = Bitmap.createBitmap(dstWidth, dstHeight, Bitmap.Config.RGB_565);
    Canvas canvas = new Canvas(desBitmap);
    Paint paint = new Paint();
//前者
    long time0 = SystemClock.elapsedRealtime();
    Matrix mMatrix = new Matrix();
    mMatrix.postScale((float) dstWidth / srcBitmap.getWidth(),
                (float) dstHeight / srcBitmap.getHeight());
    canvas.drawBitmap(srcBitmap, mMatrix, paint);
    Logger.d("Matrix time：" + (SystemClock.elapsedRealtime() - time0));
//后者
    time0 = SystemClock.elapsedRealtime();
    canvas.drawBitmap(srcBitmap,
            new Rect(0, 0, srcBitmap.getWidth(), srcBitmap.getHeight()),
            new Rect(0, 0, dstWidth, dstHeight), paint);
    Logger.d("Rect time：" + (SystemClock.elapsedRealtime() - time0));

    srcBitmap.recycle();
    desBitmap.recycle();
}
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测试结果

前者Matrix （22ms）比后者Rect（35ms）快，性能越差的设备越明显

测试结论

尽量用Matrix替换Rect

drawBitmap源码跟踪

通过8.0源码可以找到 public class Canvas extends BaseCanvas
在BaseCanvas有如下代码

public void drawBitmap(@NonNull Bitmap bitmap, @NonNull Matrix matrix, @Nullable Paint paint) {
        throwIfHasHwBitmapInSwMode(paint);
        nDrawBitmapMatrix(mNativeCanvasWrapper, bitmap.getNativeInstance(), matrix.ni(),
                paint != null ? paint.getNativeInstance() : 0);
    }
public void drawBitmap(@NonNull Bitmap bitmap, @Nullable Rect src, @NonNull Rect dst,
            @Nullable Paint paint) {
        ...
        nDrawBitmap(mNativeCanvasWrapper, bitmap.getNativeInstance(), left, top, right, bottom,
                dst.left, dst.top, dst.right, dst.bottom, nativePaint, mScreenDensity,
                bitmap.mDensity);
    }
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最终调用在 frameworks/base/core/jni/android_graphics_Canvas.cpp，其中有如下代码片段

static void drawBitmapMatrix(JNIEnv* env, jobject, jlong canvasHandle, jobject jbitmap,
                             jlong matrixHandle, jlong paintHandle) {
    const SkMatrix* matrix = reinterpret_cast<SkMatrix*>(matrixHandle);
    const Paint* paint = reinterpret_cast<Paint*>(paintHandle);
    Bitmap& bitmap = android::bitmap::toBitmap(env, jbitmap);
    get_canvas(canvasHandle)->drawBitmap(bitmap, *matrix, paint);
}
static void drawBitmapRect(JNIEnv* env, jobject, jlong canvasHandle, jobject jbitmap,
                           float srcLeft, float srcTop, float srcRight, float srcBottom,
                           float dstLeft, float dstTop, float dstRight, float dstBottom,
                           jlong paintHandle, jint screenDensity, jint bitmapDensity) {
    Canvas* canvas = get_canvas(canvasHandle);
    const Paint* paint = reinterpret_cast<Paint*>(paintHandle);
    Bitmap& bitmap = android::bitmap::toBitmap(env, jbitmap);
    if (screenDensity != 0 && screenDensity != bitmapDensity) {
        Paint filteredPaint;
        if (paint) {
            filteredPaint = *paint;
        }
        filteredPaint.setFilterQuality(kLow_SkFilterQuality);
        canvas->drawBitmap(bitmap, srcLeft, srcTop, srcRight, srcBottom,
                           dstLeft, dstTop, dstRight, dstBottom, &filteredPaint);
    } else {
        canvas->drawBitmap(bitmap, srcLeft, srcTop, srcRight, srcBottom,
                           dstLeft, dstTop, dstRight, dstBottom, paint);
    }
}
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即最终分别调用了
canvas->drawBitmap(bitmap, *matrix, paint);
canvas->drawBitmap(bitmap, srcLeft, srcTop, srcRight, srcBottom, dstLeft, dstTop, dstRight, dstBottom, paint);

在frameworks/base/libs/hwui/hwui/Canvas.cpp有如下代码

Canvas* Canvas::create_recording_canvas(int width, int height, uirenderer::RenderNode* renderNode) {
    if (uirenderer::Properties::isSkiaEnabled()) {
        return new uirenderer::skiapipeline::SkiaRecordingCanvas(renderNode, width, height);
    }
    return new uirenderer::RecordingCanvas(width, height);
}
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因此Canvas就是是RecordingCanvas，在 frameworks/base/libs/hwui/RecordingCanvas.cpp有如下代码

void RecordingCanvas::drawBitmap(Bitmap& bitmap, const SkMatrix& matrix,
                            const SkPaint* paint) {
    if (matrix.isIdentity()) {
        drawBitmap(bitmap, paint);
    } else if (!(matrix.getType() & ~(SkMatrix::kScale_Mask | SkMatrix::kTranslate_Mask))
            && MathUtils::isPositive(matrix.getScaleX())
            && MathUtils::isPositive(matrix.getScaleY())) {
        // SkMatrix::isScaleTranslate() not available in L
        SkRect src;
        SkRect dst;
        bitmap.getBounds(&src);
        matrix.mapRect(&dst, src);
        drawBitmap(bitmap, src.fLeft, src.fTop, src.fRight, src.fBottom,
                   dst.fLeft, dst.fTop, dst.fRight, dst.fBottom, paint);
    } else {
        save(SaveFlags::Matrix);
        concat(matrix);
        drawBitmap(bitmap, paint);
        restore();
    }
}
void RecordingCanvas::drawBitmap(Bitmap& bitmap, float srcLeft, float srcTop,
            float srcRight, float srcBottom, float dstLeft, float dstTop,
            float dstRight, float dstBottom, const SkPaint* paint) {
    if (srcLeft == 0 && srcTop == 0
            && srcRight == bitmap.width()
            && srcBottom == bitmap.height()
            && (srcBottom - srcTop == dstBottom - dstTop)
            && (srcRight - srcLeft == dstRight - dstLeft)) {
        // transform simple rect to rect drawing case into position bitmap ops, since they merge
        save(SaveFlags::Matrix);
        translate(dstLeft, dstTop);
        drawBitmap(bitmap, paint);
        restore();
    } else {
        addOp(alloc().create_trivial<BitmapRectOp>(
                Rect(dstLeft, dstTop, dstRight, dstBottom),
                *(mState.currentSnapshot()->transform),
                getRecordedClip(),
                refPaint(paint), refBitmap(bitmap),
                Rect(srcLeft, srcTop, srcRight, srcBottom)));
    }
}
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