Describe most of the current lesson differences

README.md

@@ -8,15 +8,70 @@ and Swift)
 ## Lessons [01 - 05](https://nehe.gamedev.net/tutorial/lessons_01__05/22004/) ##
 
 ### Lesson 01: [Creating An ~~OpenGL~~ SDL_GPU Window](https://nehe.gamedev.net/tutorial/creating_an_opengl_window_(win32)/13001/) ###
+The simplest lesson, covers creating just enough render pass to clear the
+screen.
+
 ### Lesson 02: [Creating Your First Polygon & Quad](https://nehe.gamedev.net/tutorial/your_first_polygon/13002/) ###
+The real polygonal "Hello world".
+Instead of old-school `glBegin`/`glEnd` calls we store the shapes in static
+vertex & index buffers, which are much more efficient. We also introduce our
+first shader program, which simply applies our combined model-view-projection
+matrix to the model in the vertex shader and shades the fragments white.
+
 ### Lesson 03: [Flat and Smooth Colors](https://nehe.gamedev.net/tutorial/adding_colour/13003/) ###
+In place of `glColor3f` calls; we add RGBA components to our vertex structure
+and modify our shader program to interpolate colours between vertices and
+output the result to each fragment.
+
 ### Lesson 04: [Rotating A Polygon](https://nehe.gamedev.net/tutorial/rotation/14001/) ###
+We have already been using a tiny matrix maths library in place of legacy GL's
+`gl*` matrix stack manipulation calls, we're now using `Mtx_Rotate` in place of
+`glRotatef` to apply a little animation to our previously static shapes.
+
 ### Lesson 05: [Solid Objects](https://nehe.gamedev.net/tutorial/3d_shapes/10035/) ###
+Like the original, we modify our shapes to fully bring them into the 3rd
+dimension, turning our triangle into a pyramid and our quad into a cube.
 
 ## Lessons [06 - 10](https://nehe.gamedev.net/tutorial/lessons_06__10/17010/) ##
 
 ### Lesson 06: [Texture Mapping](https://nehe.gamedev.net/tutorial/texture_mapping/12038/) ###
+Finally, we get to texture mapping. The original lesson is split between an
+older version based around `auxDIBImageLoad` from the long-deprecated
+Microsoft-only GLaux library, and an updated addendum demonstrating
+replacing its usage with [SOIL](https://github.com/SpartanJ/SOIL2). Because
+the textures are in BMP format we can use the SDL built-in `SDL_LoadBMP`,
+which lets us take advantage of the vast Surfaces API to flip the image into
+the orientation the original program expects, as well as handle pixel format
+conversion for us.
+
+We modify our pipeline and shader program to replace vertex colours with a
+texture sampler, we can now bind our texture to the render pass and draw a
+beautiful textured cube.
+
 ### Lesson 07: [Texture Filters, Basic Lighting & Keyboard Control](https://nehe.gamedev.net/tutorial/texture_filters,_lighting_&_keyboard_control/15002/) ###
+This lesson introduces multiple texture filtering styles, keyboard control for
+rotating the cube, and togglable lighting.
+
+Lighting introduces a new shader which implements just enough legacy-style
+Gouraud shading to visually match the original, which required adding a few
+magic constants to the lighting maths. As lighting is a separate shader
+program; we create a second pipeline state for it.
+
+OpenGL filtering & wrap modes are properties of a texture, NeHe chose to
+implement switching filtering modes by uploading the same texture multiple
+times and switching them at runtime. In modern graphics APIs; texture buffers
+and samplers are separate constructs, so we only need to upload one static
+texture and cycle samplers instead.
+
+The last sampler enables mip-mapping. The original lesson uses GLU's
+`gluBuild2DMipmaps` call (which can more or less be substituted with
+[`glGenerateMipmap`](https://registry.khronos.org/OpenGL-Refpages/gl4/html/glGenerateMipmap.xhtml)
+in later versions of OpenGL) to programmatically generate mip-map levels at
+startup. In SDL_GPU we can use
+[`SDL_GenerateMipmapsForGPUTexture`](https://wiki.libsdl.org/SDL3/SDL_GenerateMipmapsForGPUTexture)
+with `layer_count_or_depth` set to $floor(\log_2\max(width,height))+1$ to
+replicate OpenGL's behaviour.
+
 ### Lesson 08: [Blending](https://nehe.gamedev.net/tutorial/blending/16001/) ###
 ### Lesson 09: [Animated Scenes With Blended Textures](https://nehe.gamedev.net/tutorial/moving_bitmaps_in_3d_space/17001/) ###
 ### Lesson 10: [Loading And Moving Through A 3D World](https://nehe.gamedev.net/tutorial/loading_and_moving_through_a_3d_world/22003/) ###