Rendering from GL_TEXTURE_EXTERNAL_OES to GL_TEXTURE_2D but only one color flashes

Question

I'm pretty new to opengl-es and currently have a problem rendering my video output on a screen in Unity.

I was developing a video player project with Unity. I bought the EasyMovieTexture plugin and replaced the video player module with another open-source video player (Ijkplayer) years ago which worked fine all the time.

Now I want to replace it with newer VLC using libvlcjni. I compiled and just replaced the old Ijkplayer but it didn't work as I expected. The screen just started flashing one color from every video frame but the video was going and the audio track was normally playing too.

Screenshot - A test scene with a screen, sorry there's mistake with texcoord but only color flashing

I'd like to provide some further information hope that I can find some help (Sorry if I have some mistakes or misunderstandings):

1. As far as I know, these video player modules need a Surface (Or SurfaceTexture) in the Android layer and the video decoder will work as a data producer. The SurfaceTexture consumes data from the producer and converts it to the texture with the type of GL_TEXTURE_EXTERNAL_OES which can be directly consumed and displayed with components like TextureView. But this texture data cannot be consumed in the Unity layer unless I use a GLSL shader directly sampling my OES texture data, like this:

   // Unity shader with GLSL
   GLSLPROGRAM
       #pragma only_renderers gles3
       #include "UnityCG.glslinc"
   // ...
   // Ignoring vertex shader
   // ...
       in vec2 textureCoord;
       layout(binding = 0) uniform samplerExternalOES _MainTex;
       out vec4 fragColor;
       void main()
       {
           fragColor  = texture(_MainTex, textureCoord);
       }
   ENDGLSL
   

2. My approach was to convert the texture to GL_TEXTURE_2D with the native library which came along with the EasyMovieTexture plugin. Here I cannot provide the source code of this .so library but I've decompiled it in IDAPro and I know it can work along with GLES and render the external texture data to another 2d texture using FrameBuffer Object and external shader program.
   • Here is a random example to explain the procedure, it is NOT the accurate code from the binary: FilterFBOTexture.java
   • Though I cannot edit the .so file, luckily it was reading two external files as the shader program:

   // vertex shader
   attribute highp   vec3  inVertex;
   attribute mediump vec3  inNormal;
   attribute mediump vec2  inTexCoord;
   uniform highp   mat4  MVPMatrix;
   uniform mediump vec2 TexCoordMove;
   varying mediump vec2   TexCoord;
   
   void main()
   {
       highp vec4 vPos = vec4(0,0,0,1);
   
       vPos.x = ( inTexCoord.x * 2.0 - 1.0 );
       vPos.y = ( inTexCoord.y * 2.0 - 1.0 );
   
       gl_Position = vPos;
   
       mediump vec4 vec4Temp  = vec4(inTexCoord.x - TexCoordMove.x,inTexCoord.y - TexCoordMove.y,0,1.0);
       vec4Temp  = MVPMatrix * vec4Temp;
       vec4Temp.xyz = vec4Temp.xyz / vec4Temp.w;
       TexCoord = vec4Temp.xy;
   }
   

   // fragment shader
   #extension GL_OES_EGL_image_external : require
   uniform samplerExternalOES sTexture;
   uniform lowp float AlphaValue;
   varying mediump vec2  TexCoord;
   void main()
   {
       lowp vec4 color = texture2D(sTexture, TexCoord) ;
       color = vec4(color.rgb, color.a * AlphaValue);
       gl_FragColor = color;
   }
   

I don't know whether I have to check the vertex shader or just dive into the source code of libvlcjni so that I can correctly render my video output. Any idea will be grateful, thanks.

Update 2022-11-4:

I turned around and began to use VLC for Android.

Big appreciation to @mfkl, I created an issue days ago on the VLC repo.

https://code.videolan.org/videolan/vlc-unity/-/issues/164

The problem still remains but at least I can work for something now.