title: How to repeatedly update a uniform data for number of objects inside a single Vulkan render pass and make the update synchronized?
body: <p>I'm trying to port my OpenGL 3D game engine to Vulkan. There are large numbers of 3D objects in the game scene and each has it own attributes (model matrix, lights, etc.) and the objects are completely dynamic, which means some 3D objects may come in and others may be removed during the game play. with OpenGL, I grouped 3D object's attribute into a uniform buffer in shader (code simplified):</p>
  
  <pre><code>
      layout(std140, set = 0, binding = 0) uniform object_attrib 
      {
          vec3 light_pos;
          vec3 light_color;
          mat4 model;
          mat4 view_projection;
          ...
      } params;
  
  </code></pre>
  What I'm trying to do now is using this single uniform buffer for every 3D objects in the game scene to render them by Vulkan.
  
  <p>I'm using a single Vulkan render pass, within begin-render-pass and end-render-pass, I use a for-each loop to go over each 3D objects and do the following things to render them. See the pseudo code below.</p>
  
  <pre><code>
      vkBeginCommandBuffer(cmdBuffer, ...);
          vkCmdBeginRenderPass(cmdBuffer, ...);
              for(object3D obj : scene-&gt;objects)
              { 
                  // Step 1 - update object's uniform data by memcpy()
                  _updateUniformBuffer(obj); 
  
                  // Step 2 - build draw command for this object
                  // bind vertex buffer, bind index buffer, bind pipeline, ..., draw
                  _buildDrawCommands(obj);
              }
          vkCmdEndRenderPass(cmdBuffer, ...);
      vkEndCommandBuffer(cmdBuffer, ...);
      vkQueueSubmit(...); // Finally, submit the commands to queue to render the scene
  
  </code></pre>
  
  <p>Obviously, my solution will no work since all Vulkan commands in the buffer are executed on GPU only after vkQueueSubmit() is called. But the call to _updateUniformBuffer(obj) (by memcpy(...)) is "interleaved" with command recording and it is executed immediately and therefore the sequence are messed up and finally each object will not get its own attributes.</p>
  
  <p><b>So may question is what the solution for Vulkan to properly update uniform buffer repeatedly for each object inside a single render pass and make sure each object get its correct attribute data?</b></p>
  
  <p>Before I post this question, I tried to think about the following solutions but  none of them seems to be a good one:</p>
  
  <ul>
  <li>Using render-pass-per-object and use fence to make sure one object is completely rendered until I start rendering the next one. If there are 1000 objects, there will be 1000 render pass per frame? This is impossible.</li>
  <li>Can I submit command buffer repeatedly inside one render pass? I means I submit command buffer right after the draw command for one object is built to render the object, use fence to make sure render is done, then go to the next object. This will have a single render pass and 1000 vkQueueSubmit() calls</li>
  <li>Using dynamic uniform buffer which create a huge uniform buffer contains data for 1000 objects. It is difficult to implement since the object number are dynamic.</li>
  <li>Using push constant? It is also impossible since the max data size is only 128 bytes.</li>
  </ul>