Can you write to a Unity texture, from an iOS C plugin, "on the spot"?

Question

Say you have a low-level Unity plugin for iOS,

So in c#

using System.Runtime.InteropServices;
using AOT;

public class Teste: MonoBehaviour {

    [DllImport("__Internal")] private static extern
       void set_texture_from_unity(System.IntPtr texture, int w, int h);

have a texture, .Apply() it, and then send the pointer to the native iOS plugin:

public void ClickPassTexture() {

    tex = new Texture2D(256, 256, TextureFormat.ARGB32, false);
    tex.filterMode = FilterMode.Point;

    tex.Apply(); // ACTUALLY UPLOAD TO GPU

    someMaterial.mainTexture = tex;

    set_texture_from_unity(
       tex.GetNativeTexturePtr(), tex.width, tex.height);
}

Now the C code.

Make a texture with a few colored lines, and write it to the Unity texture.

#include <OpenGLES/ES2/gl.h>
...
void set_texture_from_unity(void *g_TextureHandle,  int w, int h)) {
    
    // make a texture with a few gray rows
    unsigned char* data = malloc( g_TextureWidth * 4 * g_TextureHeight );
    for (int i = 0; i < 1000; ++i) { data[i] = 100; }

    // now, write that to the texture pointer given to us from Unity
    GLuint gltex = (GLuint)(size_t)(g_TextureHandle);
    glBindTexture(GL_TEXTURE_2D, gltex);
    glTexSubImage2D(GL_TEXTURE_2D, 0, 0, 0,
      g_TextureWidth, g_TextureHeight, GL_RGBA, GL_UNSIGNED_BYTE, data);

    free(data);

So, in the C code, we have seemingly successfully modified the texture tex.

However.

It does not appear on screen.

Image shows complete failure to achieve anything at all.

The fine Unity doco docs.unity3d.com/Manual/NativePluginInterface.html

gives an example where the C texture change gets called by a callback in the Unity rendering chain.

However.

Can we simply make Unity take the new texture "now"?

When we want to ?

I thought this might simply work:

    set_texture_from_unity( tex.GetNativeTexturePtr(),
          tex.width, tex.height);
    // the plugin has changed the texture...
    // perhaps an Apply will change it on screen?!
    tex.Apply();

But no! (It makes no difference if you wait a few frames before the .Apply.)

You'd think .Apply would send the texture up to the gpu again, but it seems to not work.

In short, what to do to the presented code, to make it show the new texture "there and then", after the C plugin runs??

When you know a Texture has been modified by a C plugin, this on iOS, how to get Unity to display the change???

Answer 1

Solution in the "new" Unity

The (legendary) Keijiro Takahashi from Unity Japan, has posted probably the single most important thing ever posted from Unity:

https://github.com/keijiro/TextureUpdateExample

An actual example of the (new) frame-wise style texture-updating from plugin.

AND IT WORKS WITH IOS

Again, all that can be said is this pretty much the most valuable thing ever emitted from Unity! Phew!

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Regarding the question as asked, I really have never found the solution.