Step-through shader debugging has been available since DirectX 9 or earlier. Microsoft shipped a debugger called PIX and a reference shader rasterizer that PIX

Step-through shader debugging has been available since DirectX 9 or earlier. Microsoft shipped a debugger called PIX and a reference shader rasterizer that PIX would use to generate a reference image and let you step through how the shader should behave. I used it extensively. Reference rasterizers also exist for D3D11 (Windows ships with one called WARP and it's fast enough to actually use for many workloads). I think there might be one for D3D12 as well but I'm not sure. The dark ages 'printf with colors' stuff they describe is somewhat unique to GLSL and is just a smaller example of how the entire OpenGL ecosystem was horrible for debugging and not well designed. These days people can use Vulkan or D3D11 if they want an ecosystem that was designed intentionally and has good tooling (D3D12 is more comparable to Vulkan but is honestly not worth the trouble for most people yet). For a taste of what good modern graphics debugging looks like, run a modern game in Renderdoc and you can take a snapshot and then pick a random pixel on-screen and step-through debug the shader. Naturally, game consoles have even better tooling. The PS4 SDK let you step through the actual low level instructions running on the GPU (I'm not sure if they were simulating execution or not though...)