[FFmpeg] Draw a rectangular box

The easiest is to use filters

# 查看滤镜帮助
ffplay -h filter=drawbox
# 单个矩形
ffplay -i fpx.gif -vf drawbox:x=10:y=10:w=50:h=50:c=red
# 多个矩形，中间使用逗号连接
ffplay -i fpx.gif -vf drawbox:x=10:y=10:w=50:h=50:c=red,\
					  drawbox:x=100:y=10:w=50:h=50:c=rgb(0,0,255)

Now the YOLO algorithm detects the position of the object in each frame of the picture (sx, sy, s->w, s->h), and it is necessary to embed the function of drawing a rectangular box in the ffplay source code. First, please refer to ffmpeg-4.3.1/libavfilter/vf_drawbox.c.

image:

Core code (valid only for YUV format):

for (y = FFMAX(yb, 0); y < FFMIN(yb + s->h, frame->height); y++) {
    
    
    row[0] = frame->data[0] + y * frame->linesize[0];

    for (plane = 1; plane < 3; plane++)
        row[plane] = frame->data[plane] +
                frame->linesize[plane] * (y >> s->vsub);

    for (x = FFMAX(xb, 0); x < FFMIN(xb + s->w, frame->width); x++) {
    
    
        if (pixel_belongs_to_box(s, x, y)) {
    
    
            row[0][x           ] = s->yuv_color[Y];
            row[1][x >> s->hsub] = s->yuv_color[U];
            row[2][x >> s->hsub] = s->yuv_color[V];
        }
    }
}

The principle is very simple. Two for loops traverse each pixel of the rectangle, and use the pixel_belongs_to_box() function to determine whether the point is within the rectangular box. If it is, modify the color of the point. The pixel_belongs_to_box() function is as follows, s->thickness represents the thickness of the rectangular box.

static int pixel_belongs_to_box(DrawBoxContext *s, int x, int y)
{
    
    
    return (y - s->y < s->thickness) || (s->y + s->h - 1 - y < s->thickness) ||
           (x - s->x < s->thickness) || (s->x + s->w - 1 - x < s->thickness);
}

In the above core code, s->hsub and s->vsub also appear. These are chrominance sub-sampling values, which are related to the pixel format. The acquisition method is as follows. pix_fmt is the pixel format of the frame.

const AVPixFmtDescriptor *desc = av_pix_fmt_desc_get(pix_fmt);
s->hsub = desc->log2_chroma_w;
s->vsub = desc->log2_chroma_h;

====================================================================

The disadvantage is that scanning the entire rectangle causes the speed to be too slow . After all, we only draw the rectangle instead of the entire rectangle, so we make improvements:

for (y = FFMAX(yb, 0); y < FFMIN(yb + s->h, frame->height); y++) {
    
    
    row[0] = frame->data[0] + y * frame->linesize[0];

    for (plane = 1; plane < 3; plane++)
        row[plane] = frame->data[plane] +
                frame->linesize[plane] * (y >> s->vsub);

    if ((y - yb < s->thickness) || (yb + s->h - 1 - y < s->thickness)) {
    
    
        for (x = FFMAX(xb, 0); x < FFMIN(xb + s->w, frame->width); x++) {
    
    
            row[0][x           ] = s->dst_color[Y];
            row[1][x >> s->hsub] = s->dst_color[U];
            row[2][x >> s->hsub] = s->dst_color[V];
        }
    } else {
    
    
        for (x = FFMAX(xb, 0); x < xb + s->thickness; x++) {
    
    
            row[0][x           ] = s->dst_color[Y];
            row[1][x >> s->hsub] = s->dst_color[U];
            row[2][x >> s->hsub] = s->dst_color[V];
        }
        for (x = xb + s->w - s->thickness; x < FFMIN(xb + s->w, frame->width); x++) {
    
    
            row[0][x           ] = s->dst_color[Y];
            row[1][x >> s->hsub] = s->dst_color[U];
            row[2][x >> s->hsub] = s->dst_color[V];
        }
    }
}

The principle is also to scan from top to bottom, but by judging the y coordinate, the blank part in the middle of the rectangular frame is skipped, and the speed is greatly accelerated!