yuv420p scale issue with libyuv, getting wrong image out

Question

I have a raw yuv420p file at 1860x1920, fill it to cv::Mat, then use libyuv::I420Scale shrunk to a quarter of its original size，but get wired outout, for example there are some green stripes like this:

Use ffplay -f rawvideo -i yuv_image.yuv -video_size 1860x1920, the origin image here:

The scaled image is not normal, it looks like the yuv data is not aligned

What am I doing wrong, code below:

#define ALIGN32(n) (((n) >> 5) << 5)
// yuv420_image is yuv420 cv::Mat
int src_width = yuv420_image.cols, src_height = yuv420_image.rows / 3 * 2;
const uint8_t* src = yuv420_image.data;
const uint8_t* src_y = src;
const uint8_t* src_u = src_y + src_width * src_width;
const uint8_t* src_v = src_u + src_width * src_width / 4;

int dst_width = ALIGN32(src_width / kScaleRatio), dst_height = ALIGN32(src_height / kScaleRatio);
cv::Mat yuv420_scaled(dst_height * 3 / 2, dst_width,
                    CV_8UC1);
uint8_t* dst = yuv420_scaled.data;
uint8_t* dst_y = dst;
uint8_t* dst_u = dst_y + dst_width * dst_height;
uint8_t* dst_v = dst_u + dst_width * dst_height / 4;
int result = libyuv::I420Scale(src_y, src_width,
                          src_u, src_width / 2,
                          src_v, src_width / 2,
                          src_width, src_width,
                          dst_y, dst_width,
                          dst_u, dst_width / 2,
                          dst_v, dst_width / 2,
                          dst_width, dst_height,
                          libyuv::FilterModeEnum::kFilterNone);

Answer 1

The issue is mixing width and height in 3 places.

Issues:

• Mixing src_width and src_height:
  const uint8_t* src_u = src_y + src_width * src_width;
const uint8_t* src_v = src_u + src_width * src_width / 4;
  Supposed to be:

    const uint8_t* src_u = src_y + src_width * src_height;
    const uint8_t* src_v = src_u + src_width * src_height / 4;

• Mixing src_width and src_height:
  int result = libyuv::I420Scale(src_y, src_width,
src_u, src_width / 2,
src_v, src_width / 2,
src_width, src_width,
  Supposed to be:

    int result = libyuv::I420Scale(src_y, src_width,
        src_u, src_width / 2,
        src_v, src_width / 2,
        src_width, src_height,

---

For testing I created a synthetic video frame in YUV420p (I420) pixel format using FFmpeg:

ffmpeg -f lavfi -i testsrc=1860x1920:rate=1:duration=1 -vf scale=out_color_matrix=bt709:out_range=full -pix_fmt yuv420p yuv_image.yuv

---

The following code sample downscales the image to 448x480.
The code uses OpenCV for converting the result to BGR (for testing).

#include <stdio.h>
#include <stdlib.h>
#include <stdint.h>

#include "opencv2/opencv.hpp"
#include "libyuv.h"

#define ALIGN32(n) (((n) >> 5) << 5)

//Building a sample input image:
//ffmpeg -f lavfi -i testsrc=1860x1920:rate=1:duration=1 -vf scale=out_color_matrix=bt709:out_range=full -pix_fmt yuv420p yuv_image.yuv

int main()
{
    const int kScaleRatio = 4;  //Set to 4 for example

    const int width = 1860;
    const int height = 1920;
    const int stride = width;   //Assume rows are continuous

    uint8_t *frameData = new uint8_t[stride*height*3/2];  //Buffer for storing raw I420 input image.

    //Read image from file.
    FILE* f = fopen("yuv_image.yuv", "rb");
    fread(frameData, 1, stride*height*3/2, f);  //Assume rows are continuous (assume stride = width)
    fclose(f);

    //Make OpenCV Mat wrapper
    cv::Mat yuv420_image(height*3/2, width, CV_8UC1, (void*)frameData, stride);

    // yuv420_image is yuv420 cv::Mat
    int src_width = yuv420_image.cols, src_height = yuv420_image.rows / 3 * 2;
    const uint8_t* src = yuv420_image.data;
    const uint8_t* src_y = src;
    const uint8_t* src_u = src_y + src_width * src_height;       //const uint8_t* src_u = src_y + src_width * src_width;  <-- Supposed to be src_width * src_height
    const uint8_t* src_v = src_u + src_width * src_height / 4;   //const uint8_t* src_v = src_u + src_width * src_width / 4;

    int dst_width = ALIGN32(src_width / kScaleRatio), dst_height = ALIGN32(src_height / kScaleRatio);
    cv::Mat yuv420_scaled(dst_height * 3 / 2, dst_width, CV_8UC1);
    uint8_t* dst = yuv420_scaled.data;
    uint8_t* dst_y = dst;
    uint8_t* dst_u = dst_y + dst_width * dst_height;
    uint8_t* dst_v = dst_u + dst_width * dst_height / 4;

    //int result = libyuv::I420Scale(src_y, src_width,
    //    src_u, src_width / 2,
    //    src_v, src_width / 2,
    //    src_width, src_width,   //<--  Supposed to be src_width, src_height
    //    dst_y, dst_width,
    //    dst_u, dst_width / 2,
    //    dst_v, dst_width / 2,
    //    dst_width, dst_height,
    //    libyuv::FilterModeEnum::kFilterNone);

    int result = I420Scale(src_y,           //const uint8_t * src_y,
                           src_width,       //int src_stride_y,
                           src_u,           //const uint8_t * src_u,
                           src_width / 2,   //int src_stride_u,
                           src_v,           //const uint8_t * src_v,
                           src_width / 2,   //int src_stride_v,
                           src_width,       //int src_width,
                           src_height,      //int src_height,
                           dst_y,           //uint8_t * dst_y,
                           dst_width,       //int dst_stride_y,
                           dst_u,           //uint8_t * dst_u,
                           dst_width / 2,   //int dst_stride_u,
                           dst_v,           //uint8_t * dst_v,
                           dst_width / 2,   //int dst_stride_v,
                           dst_width,       //int dst_width,
                           dst_height,      //int dst_height,
                           libyuv::FilterModeEnum::kFilterNone);//enum FilterMode filtering);

    if (result != 0)
    {
        return result;
    }

    cv::Mat bgr_scaled;

    //Convert YUV420p to BGR using OpenCV (note: the conversion may not result accurate colors).
    //It looks like the is a bug in OpenCV (use COLOR_YUV420p2RGB instead of COLOR_YUV420p2BGR).
    cv::cvtColor(yuv420_scaled, bgr_scaled, cv::COLOR_YUV420p2RGB);

    //Show bgr_scaled for testing.
    cv::imshow("bgr_scaled", bgr_scaled);
    cv::waitKey();
    cv::destroyAllWindows();

    delete[] frameData;

    cv::imwrite("bgr_scaled.png", bgr_scaled);

    return 0;
}

---

Output (after converting to BGR):