This is one of the most critical sections of this console, it basically makes the GameCube, a GameCube.
The history of this console's GPU has some interesting connections: Wei Yen, the director of N64's SoC (the RCP), later founded ArtX and landed a contract with Nintendo to develop their next-gen chip: Flipper.
Super Mario Sunshine (2002).
There were lots of advancements done from the previous iteration, for instance, the subsystem was severely simplified down to a single (but powerful) core.
During the development process, ArtX got acquired by ATI, which in turn was sold to AMD six years later. Hence, this is why you see an ATI sticker stamped on the front of the case.
Architecture and design
Flipper is a complex block that handles multiple services , so let's focus on the graphics component for now (since it's the one responsible for bringing our geometry to life). We'll call this area the GPU or Graphics Engine and, if you've been reading the N64 article, just letting you know that the core is now functional out of the box, so programmers won't need to worry about injecting code to make it work. Nevertheless, there will be some interesting parts that are customisable.
Pipeline design of Flipper's GPU.
As always, in order to draw a frame on the screen, our data will be pumped through the GPU's pipeline. Data goes through lots of different components which we can group into four stages:
Database
Database stage diagrams.
The CPU and GPU communicate to each other using a fixed-length FIFO buffer in main RAM, this is a reserved portion where the CPU will write drawing commands that the GPU will read (and eventually display), this functionality is natively supported by the CPU and GPU.
Furthermore, the CPU and GPU don't have to be pointing at the same FIFO at the same time, so the CPU can fill a separate one while the GPU is reading the first one . This prevents idling.
Issuing individual commands to construct our geometry can get very tedious with complex scenes, so official libraries included tools that generated the required Display Lists (pre-compiled set FIFO commands) from the game's assets, this chunk only needs to be copied to RAM to let the GPU effectively display them.
The GPU contains a command processor which is in charge of fetching commands from FIFO.
Geometry
Vertex stage diagram using indirect mode.
Here primitives are transformed to shape accordingly for the desired scenery and prepared for rasterising. The engine uses a dedicated Vertex unit or 'VU' to accomplish this.
There are two vertex modes available to handle primitives issued through FIFO:
- Direct mode: The CPU issues each FIFO entry with all required attributes (position, normal, colour, texture coordinate, or matrix index). Useful when the data is already cached.
- Indirect mode: The FIFO entry contains an index value that specifies where the attribute information is located in RAM, so the vertex unit needs to look it up by itself. This data is structured as an array, so for the VU to traverse it, each vertex entry must specify where the array starts (base pointer), how long is each entry (stride) and at which position the vertex is (index).
Once loaded, the primitives can be transformed, clipped, lighted (each vertex will have an RGB value that can also be interpolated for Gouraud Shading purposes) and finally, projected.
Texture
Texture stage diagram using a default setup.
Now it's time to apply textures and effects to our models, and for that the GPU includes multiple units which will process our pixels. Now, this is a very sophisticated (yet quite complex) procedure, so if you find it difficult to follow, just think of it as a big assembly line that processes pixels. Having said that, there are three groups of units available:
- Four parallel Pixel units (also called 'pixel pipelines'): Rasterises our primitives (converts them to pixels). Having four units available enables to deliver up to 2x2 pixels on each cycle.
- One Texture mapping unit at the end of each Pixel unit (giving four in total): Together they process up to eight textures for our primitives (now mere pixels) at each cycle.
- It can also loop itself to merge multiple texture layers over the same primitive, this feature is called Multi-Texturing and can be used to achieve detailed textures, environment mapping (reflections) and bump mapping , for instance.
- Finally, the unit also provides an early z-buffer, mipmapping (processing a downsized texture instead, based on the level of detail) and anisotropic filtering (a welcoming improvement over the previous filters that provides greater detail with sloped textures).
- Texture Environment unit or 'TEV': A very powerful and programmable 16-stage colour blender. It basically combines multiple texels (lighting, textures and constants) to achieve an immense amount of texture effects that will be applied over our polygons.
- The unit works by receiving four texels which are then processed based on the operation requested. Afterwards, it can feed itself the resulting texels as new input, so at the next stage/cycle, the unit can perform a different type of operation over the previous result. This 'loop' can last up to 15 iterations.
- Each stage has 2^4 operations to choose from and, considering the result can be re-processed at the next stage, there are ~5.64 × 10^511 possible permutations!
- Programmers set up the TEV at runtime (meaning it can change any time) and this is crucial since it opens the door to lots of original materials and effects.
All of this is assisted by 1 MB of Texture memory (1T-SRAM type) which can be split into cache and Scratchpad memory (fast RAM). Real-time hardware decompression for SRTC (S3 Texture Compression) textures is also available to fit more textures in that single meg. of memory available.
Render
Render stage diagram.
The final stage of the rendering process includes applying some optional but useful touches to our scene:
- Fog: Blends the last colour from the TEV with a fog constant colour to effectively simulate a foggy environment.
- Z-compare: A late-stage Z-buffer. The engine will use 2 MB of embedded 1T-SRAM for Z-buffering calculations.
- Blending: Combines the colours of the current frame with the previous frame buffer.
- Dithering: As the name indicates, applies dithering over our frame.
The resulting frame is finally written to the frame buffer in the embedded 1T-SRAM, but this is still locked inside Flipper (the area is called 'Embedded Frame Buffer' or 'EFB', though it also includes the z-buffer). So, to display it on our TV, we have to copy it to the External Frame-Buffer or 'XFB' , which can be picked up the Video Interface or 'VI'. Besides, the copy process can apply effects like Antialiasing (reduces blocky edges), Deflicker (smooths sudden changes in brightness), RGB to YUV conversion (a similar format that occupies less space in memory) and Y-scaling (vertically scales the frame).
It's worth mentioning that the XFB area can also be manipulated by the CPU, this enables it to combine previously-rendered bitmaps with our recently-rendered frame; or when certain games need to render very colour-rich frames which can't fit in the EFB, so they are rendered in parts and merged by the CPU afterwards (always keeping in-sync with the VI).
Interactive comparison
Time to put all of this into perspective, check out how programmers evolved the designs of their previous games to take advantage of the new graphics capabilities of this console. Don't forget the examples are interactive!
The upgrade
The famous Mario model which had to be stripped down due to polygon counting on the previous generation got completely redesigned for this one, take a closer look at how the model evolved from plain faces to wrinkled sleeves.
Interactive model available in the modern edition
Super Smash Bros (1999) for the N64.
320 triangles.
Interactive model available in the modern edition
Super Smash Bros. Melee (2001) for the GC.
4,718 triangles.
It's really impressive how much detail has been gained in just two years, eh?
The update
In this case, Sonic Team already designed a Sonic model for their unique console, but after porting their game to the GameCube they found themselves able to add more polygons to their model, achieving better detail.
Interactive model available in the modern edition
Sonic Adventure (1999) for the DC.
1,001 triangles.
Interactive model available in the modern edition
Sonic DX (2003) for the GC.
1,993 triangles.
Creativity
As you can see from the inner working of this pipeline, graphics technology has been evolving to point that programmers can now take control of certain functions of the rendering process.
The Legend of Zelda: The Wind Waker (2003).
During the same time, PC graphics cards were starting to discard fixed-function pipelines in favour of shader cores (units that run small programs which define how pixels are operated). Flipper still contains a fixed-function GPU, however, by including components such as the TEV unit, one could argue that Nintendo provided their own shader-like solution.
I guess one of the best examples of games that exploited this new capability is The Legend of Zelda: Wind Waker which implements a unique colour/lighting technique known as Cel shading to make its textures look cartoonish.
Video output system
The video signal outputs a resolution of up to 640x480 pixels (or 768×576 px in PAL) with up to 16.7 million colours (24-bit depth). Additionally, the system could broadcast its signal in progressive mode (which has a clearer image, but not every TV may have supported it during that time).
The XFB can have multiple dimensions, so for compatibility reasons, the Video interface will try its best to display the frame by re-sampling the XFB to fit the TV screen based on the region.
Connections
The console included not one, but two video output connectors:
A/V Connections on the back.
- One named Analog A/V which is actually the good-old Multi Out. This is the most popular one.
- The PAL version of this console doesn't carry S-Video and the NTSC one doesn't provide RGB (bummer!).
- Another one called Digital A/V which sends audio and video in digital form (similarly to HDMI nowadays but using a completely different protocol!).
- Nintendo released a component cable set that connected to this socket. The same plug incorporated a video DAC and encoder to convert the digital signal into YPbPr (optimal quality).
- The cable was sold as an extra accessory and now is considered some sort of a relic!