avfilter/vf_derain.c: put all the calculation in model file.

currently, the model outputs the rain, and so need a subtraction
in filter c code to get the final derain result.

I've sent a PR to update the model file and accepted, see at
https://github.com/XueweiMeng/derain_filter/pull/3Signed-off-by: Guo, Yejun <yejun.guo@intel.com>
Signed-off-by: Steven Liu <lq@chinaffmpeg.org>

doc/filters.texi

@@ -8878,6 +8878,7 @@ delogo=x=0:y=0:w=100:h=77:band=10
 
 @end itemize
 
+@anchor{derain}
 @section derain
 
 Remove the rain in the input image/video by applying the derain methods based on
@@ -8932,6 +8933,8 @@ Note that different backends use different file formats. TensorFlow and native
 backend can load files for only its format.
 @end table
 
+It can also be finished with @ref{dnn_processing} filter.
+
 @section deshake
 
 Attempt to fix small changes in horizontal and/or vertical shift. This
@@ -9201,9 +9204,9 @@ Set the output name of the dnn network.
 
 @itemize
 @item
-Halve the red channle of the frame with format rgb24:
+Remove rain in rgb24 frame with can.pb (see @ref{derain} filter):
 @example
-ffmpeg -i input.jpg -vf format=rgb24,dnn_processing=model=halve_first_channel.model:input=dnn_in:output=dnn_out:dnn_backend=native out.native.png
+./ffmpeg -i rain.jpg -vf format=rgb24,dnn_processing=dnn_backend=tensorflow:model=can.pb:input=x:output=y derain.jpg
 @end example
 
 @item

libavfilter/vf_derain.c

@@ -100,7 +100,6 @@ static int filter_frame(AVFilterLink *inlink, AVFrame *in)
     AVFilterLink *outlink = ctx->outputs[0];
     DRContext *dr_context = ctx->priv;
     DNNReturnType dnn_result;
-    int pad_size;
 
     AVFrame *out = ff_get_video_buffer(outlink, outlink->w, outlink->h);
     if (!out) {
@@ -129,15 +128,12 @@ static int filter_frame(AVFilterLink *inlink, AVFrame *in)
     out->width  = dr_context->output.width;
     outlink->h  = dr_context->output.height;
     outlink->w  = dr_context->output.width;
-    pad_size    = (in->height - out->height) >> 1;
 
     for (int i = 0; i < out->height; i++){
         for(int j = 0; j < out->width * 3; j++){
             int k = i * out->linesize[0] + j;
             int t = i * out->width * 3 + j;
-
-            int t_in =  (i + pad_size) * in->width * 3 + j + pad_size * 3;
-            out->data[0][k] = CLIP((int)((((float *)dr_context->input.data)[t_in] - ((float *)dr_context->output.data)[t]) * 255), 0, 255);
+            out->data[0][k] = CLIP((int)((((float *)dr_context->output.data)[t]) * 255), 0, 255);
         }
     }