Jumping right in with the structural aspect of the challenge, you're doing a great job. You're making excellent use of your ellipse guides (them being similar in degree isn't a problem here since our wheels are pretty narrow and therefore won't experience that much degree shift - although getting a master ellipse template will still be very useful for Lesson 7, specifically for the "constructing to scale" approach explained there) to build out all of the necessary cross-sectional ellipses to build out the body of each wheel, and have also been quite mindful of establishing the major structures not just as shapes on a page, but also considering the side planes that define their thickness to ensure that the results are solid and believable.

To that end, the only bit of advice I want to offer is fairly minor. When you're dealing with spokes of your wheels that are simply so thin that they seem like they should be captured as singular lines (like we see here in 23), keep in mind that though making them thicker (enough so to be drawn as a shape with two edges rather than a singular line) is going to make them break away from your reference image, but in doing so it will appear more solid and structural for the simple reason that a line doesn't establish a form - at the minimum we need a shape (which itself could represent the silhouette of a 3D form) to establish the kind of solidity we're after. Breaking away from the reference - which in the context of this course is never that big of a deal, since we're using our references as a source of information and not something to reproduce at all costs - is a fair price to pay to ensure that what we're drawing still is clearly three dimensional.

Continuing onto the textural aspect of the challenge, here we get into something that - by design - ends up being a stumbling block for most students. Being as far removed from Lesson 2 where texture is first introduced, and that being a particularly unpleasant experience for most students, it's pretty normal for them to forget a lot of the concepts that are covered there.

When it comes to texture specifically - at least, how we handle it here, which is very specific to this course and what it seeks to develop in our students - we are ultimately looking at the same kind of problem that the course as a whole explores: spatial reasoning. We imply the marks we draw (you can refer to the implicit vs explicit markmaking section for more specific information on this) by drawing the shadows our textural forms cast on their surroundings, not by drawing the forms themselves (in terms of outlining them, or otherwise drawing anything about the form itself). It's the shape of the shadow itself, which is designed based on our understanding of the relationship in 3D space between the form casting it and the surface receiving it. And so, as stressed in these reminders, in this course we're never just drawing what we see. We're looking at our references, and understanding what they tell us about the forms in question, and then deciding on how to convey the relationships between them in space.

The reason we use implicit markmaking instead of explicit is fairly simple, although it's not always obvious. For example, looking at any of your wheels like number 5 here, it looks excellent floating in the void, all full of detail. But when it becomes part of an existing drawing, all of that packed detail can actually work against you by drawing the viewer's eye to it whether you want it to or not. This interferes with our ability to control composition (which is all about dictating how the viewer experiences a piece, what they look at and in which order), which while outside of the scope of this course, is still something I want to give students the tools to engage with more easily.

Explicit markmaking basically locks us into an agreement with the viewer: whatever is drawn is present, and whatever has not been drawn, is not present. And therefore to convey each textural form, we have to declare its presence explicitly. Implicit markmaking on the other hand gives us more freedom by disconnecting the marks we draw from the specifics of what is present.

By about wheel 10, I can see that you started to pick up on this weakness to your approach, and as a result you ended up trying to arbitrarily leave more and more strokes out where you tried to transition to a sparser area of detail - but this was always a matter of changing the way in which you drew the marks themselves - a change you were making purely on a 2D level - and so the choices you were making were driven by the result you wanted, but had a very tenuous structure or logic to them, because y ou were still employing explicit markmaking - drawing the textural forms themselves, rather than implicit markmaking, which involves drawing the impact (the shadow they cast) upon the surfaces around them, without drawing the forms themselves directly.

Implicit markmaking operates on a simple fact. As shown in this diagram, depending on how far the form is from the light source, the angle of the light rays will hit the object at shallower angles the farther away they are, resulting in the shadow itself being projected farther. This means that even if the textural forms that sit upon a surface do so evenly and consistently across its entirety, if we're not drawing them but rather drawing the shadows they cast, we can alter how they're drawn (making them larger and deeper in some areas, and smaller even to the point of being entirely blasted away in others) to suit our design and compositional needs. It simply gives us the excuse we need to be able to focus our visual detail in certain areas, and leave others more sparse.

The key however is two fold:

• Make sure that what you're drawing are cast shadows. The shape they take will change based on how the surface upon which they're cast sits in space, and so the shadow can describe some of that form, but what the textural marks you put down should always be cast shadows.

• Don't put marks down arbitrarily - they should always be grounded in the 3D forms that you understand to be present along that surface, even though you're not drawing them directly (which is what makes this so very difficult).

Anyway, as this challenge largely serves the purpose of rudely reminding students to consider what aspects of the course they may have allowed to slip through the cracks in the hopes that they'll go back and review those sections before carrying forward, I don't generally assign revisions for a mistake I entirely expect to occur. So! I'll go ahead and mark this challenge as complete.