use main buffer and copy data to fbo texture (opengl)

pkg/opengl/Texture.zig

@@ -162,4 +162,26 @@ pub const Binding = struct {
             data,
         );
     }
+
+    pub fn copySubImage2D(
+        b: Binding,
+        level: c.GLint,
+        xoffset: c.GLint,
+        yoffset: c.GLint,
+        x: c.GLint,
+        y: c.GLint,
+        width: c.GLsizei,
+        height: c.GLsizei,
+    ) !void {
+        glad.context.CopyTexSubImage2D.?(
+            @intFromEnum(b.target),
+            level,
+            xoffset,
+            yoffset,
+            x,
+            y,
+            width,
+            height
+        );
+    }
 };

src/renderer/OpenGL.zig

@@ -2363,26 +2363,18 @@ fn drawCustomPrograms(self: *OpenGL, custom_state: *custom.State) !void {
     // Setup the new frame
     try custom_state.newFrame();
 
-    // To allow programs to retrieve each other via a texture
-    // then we must render the next shaders to the screen fbo.
-    // However, the first shader must be run while the default fbo
-    // is attached
-    {
-        const bind = try custom_state.programs[0].bind();
-        defer bind.unbind();
-        try bind.draw();
-        if (custom_state.programs.len == 1) return;
-    }
-
-    const fbobind = try custom_state.fbo.bind(.framebuffer);
-    defer fbobind.unbind();
+    // const fbobind = try custom_state.fbo.bind(.framebuffer);
+    // defer fbobind.unbind();
 
     // Go through each custom shader and draw it.
-    for (custom_state.programs[1..]) |program| {
+    for (custom_state.programs) |program| {
         // Bind our cell program state, buffers
         const bind = try program.bind();
         defer bind.unbind();
         try bind.draw();
+
+        // copy main and custom fbo
+        try custom_state.copy();
     }
 }
 

src/renderer/opengl/custom.zig

@@ -230,6 +230,14 @@ pub const State = struct {
         };
     }
 
+    /// copy the fbo's attached texture to the backbuffer
+    pub fn copy(self: *State) !void {
+        const texbind = try self.fb_texture.bind(.@"2D");
+        errdefer texbind.unbind();
+
+        try texbind.copySubImage2D(0, 0, 0, 0, 0, @intFromFloat(self.uniforms.resolution[0]), @intFromFloat(self.uniforms.resolution[1]));
+    }
+
     pub const Binding = struct {
         vao: gl.VertexArray.Binding,
         ebo: gl.Buffer.Binding,