[FFmpeg-devel] [Patch] vf_edgedetect: properly implement double_threshold()

[Alexander Strasser]

Hi Valery!

On 2020-06-19 17:15 +0200, Valery Kot wrote:
> vf_edgedetect video filter implements Canny algorithm
> (https://en.wikipedia.org/wiki/Canny_edge_detector)
>
> Important part of this algo is the double threshold step: pixels above
> "high" threshold being kept, pixels below "low" threshold dropped,
> pixels in between kept if they are attached to "high" pixels.
>
> This is implemented in the double_threshold() function. However,
> condition to start checking attached pixels, as it is now and as it
> was in FFmpeg since 2012, only allows checking on the boundary, not
> inside the video. It is a very lucky coincidence that those boundary
> pixels are always 0, otherwise following lines would be reading
> outside of the buffer.
>
> As it is now, double_threshold() only implements "high" thresholding.
> As a result, edges are either noisy or fragmented, depending on "high"
> threshold selection; "low" threshold is simply ignored.
>
> Attached one char patch fixes this.
>
> Please review.

Looks indeed like the condition is wrong.

I say looks because I did only look and didn't do actual testing.

I hope you will answer my question anyway: Does your patch completely
fix the problem or is it sacrificing the treatment of the outer most
pixels?


  Alexander

> From b78f5960736de52d1c8e41bd598a465092c1de60 Mon Sep 17 00:00:00 2001
> From: vkot <valery.kot at kinetiq.tv>
> Date: Fri, 19 Jun 2020 16:57:13 +0200
> Subject: [PATCH] vf_edgedetect: properly implement double_threshold()
>
> ---
>  libavfilter/vf_edgedetect.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
>
> diff --git a/libavfilter/vf_edgedetect.c b/libavfilter/vf_edgedetect.c
> index a5614ea63b..df8afbd532 100644
> --- a/libavfilter/vf_edgedetect.c
> +++ b/libavfilter/vf_edgedetect.c
> @@ -294,7 +294,7 @@ static void double_threshold(int low, int high, int w, int h,
>                  continue;
>              }
>
> -            if ((!i || i == w - 1 || !j || j == h - 1) &&
> +            if (!(!i || i == w - 1 || !j || j == h - 1) &&
>                  src[i] > low &&
>                  (src[-src_linesize + i-1] > high ||
>                   src[-src_linesize + i  ] > high ||
> --