If you are at all familiar with 3D graphics, you have probably heard the term “texture” before. And if you look at virtually any instruction material on 3D graphics, they will introduce textures in the earliest parts of the work. Typically, this happens well before lighting is introduced.
This book is approximately halfway over and only now do we introduce textures. There is a good reason for this.
Consider everything you have learned up until now. You have learned how to transfer arbitrary data to vertex shaders, how to pass them to fragment shaders, and how to compute colors from them. You have learned how to transform positions of triangles and use this ability to provide a perspective projection of a world as well as to position objects and have a mobile camera. You have learned how lighting works and how to generate a lighting model. In the very last tutorial, we were able to convincingly render a mathematically perfect representation of a sphere simply by rendering two triangles.
All of this has been done without textures. Thus, the first lesson this book has to teach you about textures is that they are not that important. What you have learned is how to think about solving graphics problems without textures.
Many graphics texts overemphasize the importance of textures. This is mostly a legacy of the past. In the older days, before the availability of real programmable hardware, you needed textures to do anything of real importance in graphics rendering. Textures were used to simulate lighting and various other effects. If you wanted to do anything like per-fragment lighting, you had to use textures to do it.
Yes, textures are important for creating details in rendered images. They are important for being able to vary material parameters over a polygonal surface. And they have value in other areas as well. But there is so much more to rendering than textures, and this is especially true with programmable hardware.
A texture is a look-up table; an array. There is a lot of minutiae about accessing them, but at their core a texture is just a large array of some dimensionality that you can access from a shader. Perhaps the most important lesson you could learn is that textures are tools. Use them where appropriate, but do not let them become your primary solution to any rendering problem.