As of October 8th 2016, after roughly two years of offering free critiques, I will now be limiting my own critiques to patreon supporters ($3 or more). But don't fret - you are still welcome to freely submit your work and questions directly to the /r/ArtFundamentals subreddit to be reviewed by the community.
You guys have been asking me for some sort of a lesson on texture for ages now. Unfortunately texture is a very varied topic, and so instead of a comprehensive lesson (which wouldn't make any sense), I give you a challenge which will force you to learn what it means to capture the texture of an object. I assure you, you will cry.
Before we get into the challenge itself, I'm going to explain a little something about learning how to draw texture. There's a lot of extremely common pitfalls.
At first, everyone is guilty of this. Working from reference is paramount, but most people don't know how to use it. Most people will, at least at first, glance at their reference every now and then, between long periods of drawing. This is bad. Never go more than a few moments without looking back at your reference, and don't just look at it in general terms, study it carefully. Focus on the details you're carrying over, consider their shape, how they're arranged, how they're grouped together, and so on.
If you look away for too long, you start working from memory - in fact, most people do this without even realizing it. The problem with memory is that it's all lies - as soon as you look away, your brain gets to work simplifying everything you saw so that it can process it more easily. Our brains don't have time for all the minutiae of dense detail, it tosses most of it out. But we don't know that's happening, so we go on drawing, and end up with a laughably cartoony mess.
When you look at a complex texture in a photograph, you get overwhelmed. When you get overwhelmed, you can't even begin to imagine how to go about transferring that amount of information into your drawing. To your eyes, it looks like randomness, just lines going all over, and all you can think of is getting it over with and moving onto something that doesn't make you want to tear your eyes out. And so, you rely on randomness. You scribble, you stipple chaotically, you zigzag all over. And I break your fingers, because randomness spits in the face of everything these lessons espouse.
Every single texture, everything in nature follows a pattern, it follows a flow, it follows some sort of rhythm. It is not immediately apparent, but if you take your time, if you look at it bit by bit and just let yourself relax, you'll start to see it. Make sure you every mark you put down is related to some plan, some scheme, some observation you've made. Now so long as you promise to do that, I promise to stop writing in triad.
Depending on the reason for one's simplifying, this can just be a repetition of the first point (working from memory). There is another cause for this, however.
Ideally, when drawing texture you want to have control over how much detail you include - you want to construct clear focal points to draw the viewer's eye, and build up rest areas elsewhere. Learning to imply detail is an important skill - unfortunately many people try to jump into it far too early, before really ever learning what it means to observe, study and capture the wealth of information a texture contains. If you can't do that, then when it comes down to organize all of that information so it can be presented nicely, you're not really going to have much information to present. Because of this, your textures will appear awkward and clumsy.
This is the opposite of the previous point, and it is in many ways the goal when you start off. You want to be focusing entirely on capturing all of the detail contained in your reference images, without sorting through it or attempting to organize it. The result is, however, a really awful image too jam-packed with information that to look at it is to have your eyes poked by hot needles. It's necessary to go through this phase, but once you've learned to capture that wealth of information, then you have to start thinking about how to organize it all, and how to use the texture itself as a tool for your own purposes (which are visually communicating something), rather than being a slave to it.
The easiest way to leverage texture is to use it as a way to transition between areas of solid black, and solid white. The fact that we're using felt tip pens (also called fineliners in some parts of the world) is valuable here, because it forces us to work in such stark values. The only way we can achieve any sort of gradation is by placing bits of black and bits of white very close together, in varying densities. Lots of little lines crammed together will appear darker, and as it gets more sparse, it appears lighter.
Our texture patterns determine how we'll arrange these black and white marks, so we'll be placing the majority of our texture in the mid-tone areas of whatever we're drawing. That is, the transition areas between light and shadow. This means that we're not covering the entire thing in texture (which would be grossly overwhelming), we're just placing it in key areas. This sort of texturing relies very heavily on implying the presence of texture without explicitly drawing it all over the place. It's easy on the eyes, easy on your cramping hands, and easy on your ink budget.
The title says most of it. Textures themselves are made up of little forms, and the lines and marks that we draw are in fact not lines - they're shadows, and shadows are cast as shapes. This means that the lines in your texture can get very broad and even merge with other lines to create large areas of solid black. Do not be afraid to push the blacks in your textures.
I'd like to reiterate the point I made above - when you learn to draw texture, you do so in two steps. You do not move onto the second step until you're damn well good and ready. First, you learn to truly see all of the information contained in a reference image without getting overwhelmed, and to carry it all over into your drawing. The texturing you do during this phase is going to be very noisy, very distracting, and won't make for overly beautiful pictures (although they can be impressive in their own right). Next, you move onto figuring out how to organize all of that detail and information. Instead of your decisions being governed by your reference image, your reference becomes a tool that you can apply wherever you feel it will do the most good.
This 25 texture challenge will tackle both steps, though you'll likely need more mileage than that.
Remember that you should be doing this exercise with a felt tip pen. If you're having trouble filling in the dark areas, a black brush pen can also be very useful.
Your challenge pages will be arranged into a very specific layout, as shown here. You'll have 25 rows (obviously broken up onto different pages), each row will be roughly 2.5 inches tall, starting with a square of that height and width on the left. The rest of the width of the page (I'm recommending 8.5x11 printer paper, you'll have to change the sizes around if you're working in paper of a different size) will be split into two rectangles more or less of equal size.
Notice how the rectangle on the end there has a bit of a black bar on its left side - make sure you fill this in with solid black. It doesn't need to be too thick, just enough to be significant.
Each row is going to pertain to a different texture, and is going to require a different set of reference photos. You can use one, but it's better to use several photos of the same sort of thing, just to arm yourself with that much more visual information to draw from. Keep in mind that when we say texture, we're referring to a very specific thing.
Texture is the stuff that wraps around your major forms. Technically texture itself is made up of small forms, and the marks you see are all made up of the shadows cast by those forms (as mentioned in the pitfalls listed above), but they're at such a scale that they are not impacted by any sort of perspective distortion.
For example, "bricks" are not a texture. Bricks are at best, a pattern. The material of the bricks, however, with all their pocks and marks and grit, is certainly a texture. Similarly, "nose" is not a texture, but skin (complete with pores and hairs and zits and whatever else) is.
On the left, I've chosen a photograph of a lizard (an iguana? I don't know), and I'm going to be focusing on the texture of its scales.
The square on the left is to be filled in with the texture as you observe it, with all of its glorious detail, noise and density. Cram it all in there, don't try to imply any of it. Be explicit. This will take a long time, and it will be arduous. Worse still, you're going to be doing it 25 times, for 25 different textures. Space it out, maybe one a day or something, and spend the rest of your time on other things.
Remember, as I've mentioned twice now, the marks you're putting down are the shadows cast by the forms that make up the texture. Don't be afraid to expand them beyond lines, and fill in little areas of solid black.
The following rectangle is an easy one - just use it to write your notes and observations, all the things that you learned about that particular texture. Feel free to use words, little doodlesketches, whatever you like. Just don't write for the sake of writing, I hate seeing that. What you put down there should be of value to you, and should be something you can work with later on. One important thing to observe is how exactly the light plays off all the little forms in your texture. How much shadowplay do you see, and how does it manifest? Furthermore, taking notes on how the shapes and forms are arranged, grouped and clustered is also important.
This part is not to be done until you've done the last two steps for ALL 25 ROWS. At this point, you should have gotten a decent amount of mileage purely observing and studying textures, now we get to learning how to organize it and use it for our own purposes.
The left side of the texture has a thick black bar on it. Imagine that the right side has an equally thick white bar, where no lines may be drawn. In between these two bars, I want you to use your texture to transition from black to white, left to right. On the left, start off very dense, and make use of solid black areas that bleed into your texture to help make the transition from the black bar seamless. As you move towards the right, decrease the density of your texture. In the case of these scales, you'll see that I stopped closing my scales off, and started letting them bleed into one another.
The best way to think about this is as a photograph that's been overexposed, so the lights are dominating the image and blasting out the darks. Since all of our lines are just shadows, you can obliterate them if you apply enough light - unlike lines in a comicbook, which are more or less unassailable.
Again, I really want to make this clear - this is a lot of work. Don't even think about trying to do it in one sitting (as if such a thing were even possible). 25 may not sound like a lot in the face of the 250 boxes, but it is a tonne when you consider just how much detail I'm expecting you to cram into each texture square. Since this is not a required lesson, nothing else depends on your completion of this. It's not holding you back from moving onto anything else. So, work on this slowly, while you work on other things. Spreading it over a long period of time is likely going to allow you to benefit more from it, anyway.