What can we learn from GPU Frame Captures: Stardew Valley

If you read my last post on frame captures we discussed how GPU frame captures can be used to analyse performance, and determine how a scene is rendered for a specific game. 

It also gives insight into what the game is doing right (from a performance perspective) and how it can be improved.

In the last post we looked into Europa Universalis 4 where we found an astronomical amount of draw-calls, strange scaling behaviour and sub-optimal texture packing to render a relatively simple (although quite large) world.

In this post we are looking at a retro game for comparison. Many people who havent kept up with modern graphics development may believe that modern "retro" looking games use the same old rendering techniques that used to be mandatory on the old consoles - this isn't the case!

Stardew valley mimics the aesthetic of old SNES games like Harvest Moon but the process by which it draws the scene makes it more dynamic, smooth and pretty doing many things that would have been impossible back then to make a great experience now.

Rendering Pattern

The rendering pattern is very simple for this 2D game. It follows the expected patternn of drawing the floor and background followed by a distance sorted order of objects. In a 2D game with this view, the objects "further away" are those higher-up the screen as they will not obscure those further down. This allows the renderer to not have to worry about depth testing.

The full pattern for rendering as I could figure it out from a capture appears to be:


In this pipeline the only steps which really take any time are the two full screen passes, they are the initial ground rendering after the screen clear, and the full-screen weather effects- in our case, snow.

The initial ground rendering is a simple tiled geometry representing the game world grid and each grid cell has UV coordinates to map to a cell in the bound ground textures. 

The snow rendering is similar but mapping to a different section of the texture as shown below:

The snow is then animated by updating which cell is read for each game-world quad each frame. Notice that the background for the snow texture is not black, it is a dark grey. This is what adds the hazy effect to the scene as the texture is not directly written to the scene as the other blocks have been, it is an additive effect (D3DBLENDOP_ADD) resulting in softening of the image.

All in all it produces a quite nice, and retro effect - even though this approach might be too intensive for most real "retro" games.


Another interesting feature of this game is that it lets you customise how your character looks. Instead of using those options to generate a new sprite sheet for rendering your character, the game instead keeps all the possible options in memory and builds up the character chunk by chunk based on your choice from these options, each frame.


This is a neat and simple trick that avoids having to create a new texture when a game is loaded - but limits the extension and number of choices for the player as adding any new options increases the memory usage of the game through the entire run-time, although this is probably trivial on a game of this scale.

So how do we improve on this?

A game of this scale and already tiny frame rendering times (<2ms) there isnt really any need to improve anything. Unless you were running an incredibly outdated machine this game will run incredibly well.

However, if we were thinking of running this game on a very limited device we would probably want to consider the techniques that were used on the actual old retro games.

For starters, the majority of the scene (with an exception of the weather effects) doesn't change frame to frame - and those bits which do are somewhat predictable. There is no reason why parts of the scene couldnt be rewritten as things or the camera move and otherwise kept the same - this would infact throw out a majority of the draw-calls when the camera is stationary. The draw backs to this approach is that any mistakes in the implementation have really ugly artefacts.

This game also falls prey to the same issues as EU4 when it comes to texture sizes (although most are trivial in this case making the slight error not too big of a problem).  There are a number of large textures in this game that are just slightly above a power of two texture size - resulting in some wasted memory.

Otherwise, this frame capture is incredibly nice and easy to investigate and navigate.