I don't think I understand how the render method works.
From what I read online / saw on youtube tutorials, I gathered that the render method is a looping function that you can use to keep your game updated and read users' inputs. When you want to draw something in the screen you do so in this method, and before drawing anything you clear the screen.
But if I want to draw something stationary, wouldn't it be better to draw it outside the render method (so the computer doesn't have to clear the screen and redraw everything many times)? What am I missing?

CodePudding user response：

You can't draw outside the render method, because it's the method called on the OpenGL thread. You can't draw to OpenGL from other threads.

If your entire screen is frequently static for more than a few ms, which might be the case in something like a card game, you can disable the continuous refreshing of the screen using Gdx.graphics.setContinuousRendering(false). This will save energy because the GPU and CPU won't have to work as hard. See here for instructions on using it.

As for why this typically isn't done:

Think of a "frame buffer" as a bitmap image in memory that can be drawn to and then shown on screen. Most mobile rendering is done with double-buffering. This means while one buffer is being drawn to (the back buffer), the front buffer that was drawn to on the previous frame is being rendered directly to the screen. They swap back and forth each frame for faster rendering.

The way graphics work is that each item that's drawn modifies the color of pixels in the frame buffer where the item is drawn. Items may be masked or be translucent, and these will alter the color of pixels behind them. The original color of those pixels are lost once something is drawn in front and modifies them

So, even if some item on your screen is static, if there are other items on the screen that move or change color, and they overlap the static item (either in front of or behind), you still have to redraw the whole screen every time anything changes.

If one area of the screen is static and never overlapped by dynamic objects, and the dynamic objects are contained entirely within a rectangle, you could potentially set a viewport that limits drawing to the dynamic area and avoid clearing the whole screen. However, GPUs are designed specifically to be efficient at clearing the whole screen. Because of double-buffering, if you don't clear the whole screen, it's actually likely to harm performance because if you don't clear it, the contents of the screen have to be copied between the front and back buffers before you can start drawing the next frame.

Another strategy is if you have a static object that is very complicated and taxing for the GPU to draw because it has lots of layers or a complicated shader, you could draw it to an off-screen frame buffer object (FrameBuffer class in libGDX) one time, and then render that FrameBuffer's texture to the screen on each frame like a sprite.