opencv - 针对 ARM 优化的 FAST 计算

Question

我想通过使用 arm neon 库在 ARM cortex a8 上实现我发现的关于 5ms ORB 特征计算的论文。但我已经在为 FAST 特征检测而苦苦挣扎。所以我尝试实现的论文你可以找到 here .所以首先我不确定 Bright 和 Dark 约束。因此，根据我的理解，如果中心像素周围有 9 个较暗或 9 个较亮的像素，您必须检查 FAST。所以我检查两者。但是现在我遇到了一个问题，如果没有最终的移位操作来计算它是否是一个角，那么我的实现平均已经花费了 3 倍的时间，然后是 opencv 的整个过程的平均计算。到目前为止，这是我的代码，也许有人可以指出我可以对其进行的一些优化。

        //detect with opncv
        Clock::time_point t0 = Clock::now();
        detectors[y]->detect(img, ocv_kps);
        Clock::time_point t1 = Clock::now();

        vector<Point2f> my_kps;
        //threshhold for FAST
        const uchar th = 8;

        int b_cnt = 0;
        int d_cnt = 0;
        //array with four possible corners to be processed in parallel
        uint32_t id_arr[4];
        uint32_t ib_arr[4];

        Clock::time_point t01 = Clock::now();
        for (int i = 3; i < img.rows - 3; i++) {
            //get pointer to seven Image rows three above and three below center and center itself
            const uchar* Mt3 = img.ptr<uchar>(i - 3);
            const uchar* Mt2 = img.ptr<uchar>(i - 2);
            const uchar* Mt1 = img.ptr<uchar>(i - 1);
            const uchar* Mc = img.ptr<uchar>(i);
            const uchar* Mb1 = img.ptr<uchar>(i + 1);
            const uchar* Mb2 = img.ptr<uchar>(i + 2);
            const uchar* Mb3 = img.ptr<uchar>(i + 3);
            for (int j = 3; j < img.cols - 3; j++) {
                const uchar j3 = j + 3;
                const uchar j2 = j + 2;
                const uchar j1 = j + 1;
                const uchar jn3 = j - 3;
                const uchar jn2 = j - 2;
                const uchar jn1 = j - 1;

                 //image values for center left right top and bottom intensity of pixel
                const uchar c = Mc[j];
                const uchar l = Mc[jn3];
                const uchar r = Mc[j3];
                const uchar t = Mt3[j];
                const uchar b = Mb3[j];

                //threshold for bright FAST constraint
                const uchar thb = c + th;

                //bools for bright constraint
                const bool cbt = t > thb;
                const bool cbb = b > thb;
                const bool cbl = l > thb;
                const bool cbr = r > thb;

                 uchar mt3;
                 uchar mt3n;
                 uchar mt2;
                 uchar mt2n;
                 uchar mt1;
                 uchar mt1n;
                 uchar mb3;
                 uchar mb3n;
                 uchar mb2;
                 uchar mb2n;
                 uchar mb1;
                 uchar mb1n;
                bool bc = false;
                //pre test do we have at least two points which fulfill bright constraint
                if ((cbl && cbt) || (cbt && cbr) || (cbr && cbb)
                        || (cbb && cbl)) {
                    bc = true;
                    //get rest of image intensity values of circle
                    mt3 = Mt3[j1];
                    mt3n = Mt3[jn1];
                    mt2 = Mt2[j2];
                    mt2n = Mt2[jn2];
                    mt1 = Mt1[j3];
                    mt1n = Mt1[jn3];
                    mb3 = Mb3[j1];
                    mb3n = Mb3[jn1];
                    mb2 = Mb2[j2];
                    mb2n = Mb2[jn2];
                    mb1 = Mb1[j3];
                    mb1n = Mb1[jn3];

                    //values for bright constrain
                    ib_arr[b_cnt] = cbt | ((mt3) > thb) << 1
                            | ((mt2) > thb) << 2 | ((mt1) > thb) << 3
                            | (cbr << 4) | ((mb1) > thb) << 5
                            | ((mb2) > thb) << 6 | ((mb3) > thb) << 7
                            | cbb << 8 | ((mb3n) > thb) << 9
                            | ((mb2n) > thb) << 10 | ((mb1n) > thb) << 11
                            | (cbl) << 12 | ((mt1n) > thb) << 13
                            | ((mt2n) > thb) << 14 | ((mt3n) > thb) << 15
                            | (cbt) << 16 | ((mt3) > thb) << 17
                            | ((mt2) > thb) << 18 | ((mt1) > thb) << 19
                            | (cbr) << 20 | ((mb1) > thb) << 21
                            | ((mb2) > thb) << 22 | ((mb3) > thb) << 23;
                    b_cnt++;
                    //if we have four possible corners in array check if they are corners
                    if (b_cnt == 4) {
                        uint32x2x4_t IB = vld4_u32(ib_arr);
                        /*
                         * here the actual shift operation would take place
                         */
                        b_cnt = 0;
                    }
                }

                //threshold for dark constraint
                const uchar thd = c - th;
                //bools for dark constraint
                const bool cdl = l < thd;
                const bool cdr = r < thd;
                const bool cdt = t < thd;
                const bool cdb = b < thd;
                //pre test do we have at least two points which fulfill dark constraint
                if ((cdl && cdt) || (cdt && cdr) || (cdr && cdb)
                        || (cdb && cdl)) {
                    //if bright pre test failed intensity values are not initialised
                    if (!bc) {
                        //get rest of image intensity values of circle
                        mt3 = Mt3[j1];
                        mt3n = Mt3[jn1];
                        mt2 = Mt2[j2];
                        mt2n = Mt2[jn2];
                        mt1 = Mt1[j3];
                        mt1n = Mt1[jn3];
                        mb3 = Mb3[j1];
                        mb3n = Mb3[jn1];
                        mb2 = Mb2[j2];
                        mb2n = Mb2[jn2];
                        mb1 = Mb1[j3];
                        mb1n = Mb1[jn3];
                    }
                    //bool values for dark constrain
                    id_arr[d_cnt] = cdt | ((mt3) < thd) << 1
                            | ((mt2) < thd) << 2 | ((mt1) < thd) << 3
                            | (cdr) << 4 | ((mb1) < thd) << 5
                            | ((mb2) < thd) << 6 | ((mb3) < thd) << 7
                            | (cdb) << 8 | ((mb3n) < thd) << 9
                            | ((mb2n) < thd) << 10 | ((mb1n) < thd) << 11
                            | (cdl) << 12 | ((mt1n) < thd) << 13
                            | ((mt2n) < thd) << 14 | ((mt3n) < thd) << 15
                            | (cdt) << 16 | ((mt3) < thd) << 17
                            | ((mt2) < thd) << 18 | ((mt1) < thd) << 19
                            | (cdr) << 20 | ((mb1) < thd) << 21
                            | ((mb2) < thd) << 22 | ((mb3) < thd) << 23;
                    d_cnt++;
                    //if we have four possible corners in array check if they are corners
                    if (d_cnt == 4) {
                        uint32x2x4_t IA = vld4_u32(id_arr);
                        /*
                         * here the actual shift operation would take place
                         */
                        d_cnt = 0;
                    }
                    int h = cdt;

                }
            }
        }
        Clock::time_point t11 = Clock::now();
        cout << "my algorithm found " << my_kps.size()
                << " and ocv found " << ocv_kps.size() <<  endl;

        microseconds ms1 = std::chrono::duration_cast < microseconds
                > (t1 - t0);
        microseconds ms2 = std::chrono::duration_cast < microseconds
                > (t11 - t01);

        rs.Push((double) ms2.count());
        cout << "my algorithm duration " << ms2.count()
                << " and ocv duration is " << ms1.count()  << endl;

Answer 1

我有一个 ORB 提取器，它在树莓派上以 30fps 的速度运行。

https://github.com/0xfaded/pislam

优化真的是一门魔法，更糟糕的是，ARM 从未发布过 a53 的优化指南。我们拥有的最好的是 a57，它可能具有类似的 NEON 单元。

我真的不能在这里提供完整的答案，但我会分享一些我的过程。

我的 FAST 提取器的第一部分加载测试像素环并将它们转换为 16 位向量，就像您的代码所做的那样。我没有直接编写 asm，而是使用了 gcc 内在函数。不过，我确保 gcc:

1. 没有将任何寄存器溢出到堆栈
2. 为每次比较发出最少数量的指令

您会注意到第一个比较没有用掩码隔离它的位，掩码应该是 0x80。这释放了一个寄存器，否则它会保持一个常量，并且它给 gcc 提供了足够的回旋余地，不会溢出寄存器。

您还会注意到一些相当可怕的内在用法:

  d0 = vbslq_u8(vdupq_n_u8(0x40u), vcgeq_u8(test, dark), d0);
  l0 = vbslq_u8(vdupq_n_u8(0x40u), vcleq_u8(test, light), l0);

这相当于

  d0 |= test >= dark & 0x40;
  l0 |= test >= light & 0x40;

Gcc 会愉快地编译后者，但会发出 1.5 倍的指令。

第二部分是在 16 位向量上进行 FAST-9 测试。下面编译为 16 条指令，但我花了将近一个月的时间断断续续地想出来。

  uint8x16_t t0 = vtstq_u8(d0, d1);
  uint8x16_t t1 = vtstq_u8(d0, d1);

  t0 = vbslq_u8(t0, l0, d0);
  t1 = vbslq_u8(t1, l1, d1);

  uint8x16_t cntLo = vclzq_u8(t0);
  uint8x16_t testLo = t1 << (cntLo - 1);
  asm("vceq.u8  %q0, %q0, #0" : [val] "+w" (testLo));

  uint8x16_t cntHi = vclzq_u8(t1);
  uint8x16_t testHi = t0 << (cntHi - 1);
  asm("vceq.u8  %q0, %q0, #0" : [val] "+w" (testHi));

  uint8x16_t result = (cntLo & testLo) | (cntHi & testHi);
  result = vtstq_u8(result, result);

令人恼火的是，gcc 不会将 testLo == 0 编译为 vceq.u8 %q0, %q0, #0，这是用于与常数零。我最终手动插入了这些，从而省去了另外几条指令。

希望能提供一些见解。 Fast.h