这不是一个简单的问题，请通读!

我想操作一个 JPEG 文件并将其再次保存为 JPEG。问题是，即使没有操作，也会有显着的(可见的)质量损失。
问题 :我缺少什么选项或 API 才能在没有质量损失的情况下重新压缩 JPEG(我知道这不太可能，但我认为我在下面描述的不是可接受的伪像水平，尤其是质量 = 100)。

控制

我将它加载为 Bitmap从文件:

BitmapFactory.Options options = new BitmapFactory.Options();
// explicitly state everything so the configuration is clear
options.inPreferredConfig = Config.ARGB_8888;
options.inDither = false; // shouldn't be used anyway since 8888 can store HQ pixels
options.inScaled = false;
options.inPremultiplied = false; // no alpha, but disable explicitly
options.inSampleSize = 1; // make sure pixels are 1:1
options.inPreferQualityOverSpeed = true; // doesn't make a difference
// I'm loading the highest possible quality without any scaling/sizing/manipulation
Bitmap bitmap = BitmapFactory.decodeFile("/sdcard/image.jpg", options);

bitmap.compress(PNG, 100/*ignored*/, new FileOutputStream("/sdcard/image.png"));

// 100% still expected lossy, but not this amount of artifacts
bitmap.compress(JPEG, 100, new FileOutputStream("/sdcard/image.jpg"));

最佳答案

经过一番调查，我找到了罪魁祸首:Skia 的 YCbCr 转换。可在 TWiStErRob/AndroidJPEG 中找到 Repro、调查代码和解决方案.

发现

在没有得到对这个问题的积极回应(都不是来自 http://b.android.com/206128 )之后，我开始深入挖掘。我发现了许多半知半解的 SO 答案，这极大地帮助了我发现点点滴滴。一个这样的答案是 https://stackoverflow.com/a/13055615/253468这让我知道 YuvImage它将 YUV NV21 字节数组转换为 JPEG 压缩字节数组:

YuvImage yuv = new YuvImage(yuvData, ImageFormat.NV21, width, height, null);
yuv.compressToJpeg(new Rect(0, 0, width, height), 100, jpeg);

private static final int JSAMPLE_SIZE = 255 + 1;
private static final int CENTERJSAMPLE = 128;
private static final int SCALEBITS = 16;
private static final int CBCR_OFFSET = CENTERJSAMPLE << SCALEBITS;
private static final int ONE_HALF = 1 << (SCALEBITS - 1);

private static final int[] rgb_ycc_tab = new int[TABLE_SIZE];
static { // rgb_ycc_start
    for (int i = 0; i <= JSAMPLE_SIZE; i++) {
        rgb_ycc_tab[R_Y_OFFSET + i] = FIX(0.299) * i;
        rgb_ycc_tab[G_Y_OFFSET + i] = FIX(0.587) * i;
        rgb_ycc_tab[B_Y_OFFSET + i] = FIX(0.114) * i + ONE_HALF;
        rgb_ycc_tab[R_CB_OFFSET + i] = -FIX(0.168735892) * i;
        rgb_ycc_tab[G_CB_OFFSET + i] = -FIX(0.331264108) * i;
        rgb_ycc_tab[B_CB_OFFSET + i] = FIX(0.5) * i + CBCR_OFFSET + ONE_HALF - 1;
        rgb_ycc_tab[R_CR_OFFSET + i] = FIX(0.5) * i + CBCR_OFFSET + ONE_HALF - 1;
        rgb_ycc_tab[G_CR_OFFSET + i] = -FIX(0.418687589) * i;
        rgb_ycc_tab[B_CR_OFFSET + i] = -FIX(0.081312411) * i;
    }
}

static void rgb_ycc_convert(int[] argb, int width, int height, byte[] ycc) {
    int[] tab = LibJPEG.rgb_ycc_tab;
    final int frameSize = width * height;

    int yIndex = 0;
    int uvIndex = frameSize;
    int index = 0;
    for (int y = 0; y < height; y++) {
        for (int x = 0; x < width; x++) {
            int r = (argb[index] & 0x00ff0000) >> 16;
            int g = (argb[index] & 0x0000ff00) >> 8;
            int b = (argb[index] & 0x000000ff) >> 0;

            byte Y = (byte)((tab[r + R_Y_OFFSET] + tab[g + G_Y_OFFSET] + tab[b + B_Y_OFFSET]) >> SCALEBITS);
            byte Cb = (byte)((tab[r + R_CB_OFFSET] + tab[g + G_CB_OFFSET] + tab[b + B_CB_OFFSET]) >> SCALEBITS);
            byte Cr = (byte)((tab[r + R_CR_OFFSET] + tab[g + G_CR_OFFSET] + tab[b + B_CR_OFFSET]) >> SCALEBITS);

            ycc[yIndex++] = Y;
            if (y % 2 == 0 && index % 2 == 0) {
                ycc[uvIndex++] = Cr;
                ycc[uvIndex++] = Cb;
            }
            index++;
        }
    }
}

static byte[] compress(Bitmap bitmap) {
    int w = bitmap.getWidth();
    int h = bitmap.getHeight();
    int[] argb = new int[w * h];
    bitmap.getPixels(argb, 0, w, 0, 0, w, h);
    byte[] ycc = new byte[w * h * 3 / 2];
    rgb_ycc_convert(argb, w, h, ycc);
    argb = null; // let GC do its job
    ByteArrayOutputStream jpeg = new ByteArrayOutputStream();
    YuvImage yuvImage = new YuvImage(ycc, ImageFormat.NV21, w, h, null);
    yuvImage.compressToJpeg(new Rect(0, 0, w, h), quality, jpeg);
    return jpeg.toByteArray();
}

关于android - 如何在Android上以最少的质量损失将位图压缩为JPEG？，我们在Stack Overflow上找到一个类似的问题： https://stackoverflow.com/questions/36487971/