Up to this point we have been drawing super simple shapes. While we can make a game with just triangles, trying to draw highly detailed objects would massively limit what devices could even run our game. However, we can get around this problem with **textures**.
Textures are images overlayed on a triangle mesh to make it seem more detailed. There are multiple types of textures such as normal maps, bump maps, specular maps and diffuse maps. We're going to talk about diffuse maps, or more simply, the color texture.
We'll use the [image crate](https://crates.io/crates/image) to load our tree. We already added to our dependencies in the first section, so all we have to do is use it.
In `State`'s `new()` method add the following just after creating the `swap_chain`:
Here we grab the bytes from our image file and load them into an image which is then converted into a `Vec` of rgba bytes. We also save the image's dimensions for when we create the actual `Texture`.
The `Texture` struct has no methods to interact with the data directly. However, we can use a method on the `queue` we created earlier called `write_texture` to load the texture in. Let's take a look at how we do that:
The old way of writing data to a texture was to copy the pixel data to a buffer and then copy it to the texture. Using `write_texture` is a bit more efficient as it uses one less buffer - I'll leave it here though in case you need it.
The `bytes_per_row` field needs some consideration. This value needs to be a multiple of 256. Check out [the gif tutorial](../../showcase/gifs/#how-do-we-make-the-frames) for more details.
Now that our texture has data in it, we need a way to use it. This is where a `TextureView` and a `Sampler` come in. A `TextureView` offers us a *view* into our texture. A `Sampler` controls how the `Texture` is *sampled*. Sampling works similar to the eyedropper tool in GIMP/Photoshop. Our program supplies a coordinate on the texture (known as a *texture coordinate*), and the sampler then returns the corresponding color based on the texture and some internal parameters.
The `address_mode_*` parameters determine what to do if the sampler gets a texture coordinate that's outside of the texture itself. We have a few options to choose from:
The `mag_filter` and `min_filter` options describe what to do when a fragment covers multiple pixels, or there are multiple fragments for a single pixel. This often comes into play when viewing a surface from up close, or from far away.
*`Linear`: Attempt to blend the in-between fragments so that they seem to flow together.
*`Nearest`: In-between fragments will use the color of the nearest pixel. This creates an image that's crisper from far away, but pixelated upc close. This can be desirable, however, if your textures are designed to be pixelated, like in pixel art games, or voxel games like Minecraft.
Mipmaps are a complex topic, and will require [their own section in the future](/todo). For now, we can say that `mipmap_filter` functions similar to `(mag/min)_filter` as it tells the sampler how to blend between mipmaps.
I'm using some defaults for the other fields. If you want to see what they are, check [the wgpu docs](https://docs.rs/wgpu/0.6.0/wgpu/struct.SamplerDescriptor.html).
All these different resources are nice and all, but they don't do us much good if we can't plug them in anywhere. This is where `BindGroup`s and `PipelineLayout`s come in.
A `BindGroup` describes a set of resources and how they can be accessed by a shader. We create a `BindGroup` using a `BindGroupLayout`. Let's make one of those first.
Our `texture_bind_group_layout` has two entries: one for a sampled texture at binding 0, and one for a sampler at binding 1. Both of these bindings are visible only to the fragment shader as specified by `FRAGMENT`. The possible values for this field are any bitwise combination of `NONE`, `VERTEX`, `FRAGMENT`, or `COMPUTE`. Most of the time we'll only use `FRAGMENT` for textures and samplers, but it's good to know what else is available.
Looking at this you might get a bit of déjà vu! That's because a `BindGroup` is a more specific declaration of the `BindGroupLayout`. The reason why they're separate is it allows us to swap out `BindGroup`s on the fly, so long as they all share the same `BindGroupLayout`. For each texture and sampler we create, we need to create its own `BindGroup`.
Remember the `PipelineLayout` we created back in [the pipeline section](/beginner/tutorial3-pipeline#how-do-we-use-the-shaders)? Now we finally get to use it! The `PipelineLayout` contains a list of `BindGroupLayout`s that the pipeline can use. Modify `render_pipeline_layout` to use our `texture_bind_group_layout`.
There's a few things we need to change about our `Vertex` definition. Up to now we've been using a `color` attribute to set the color of our mesh. Now that we're using a texture, we want to replace our `color` with `tex_coords`. These coordinates will then be passed to the `Sampler` to retrieve the appropriate color.
Since our `tex_coords` are two dimensional, we'll change the field to take two floats instead of three.
With our new `Vertex` structure in place it's time to update our shaders. We'll first need to pass our `tex_coords` into the vertex shader and then use them over to our fragment shader to get the final color from the `Sampler`. Let's start with the vertex shader:
Now that we have our vertex shader outputting our `tex_cords`, we need to change the fragment shader to take them in. With these coordinates, we'll finally be able to use our sampler to get a color from our texture.
You'll notice that `t_diffuse` and `s_diffuse` are defined with the `uniform` keyword, they don't have `in` nor `out`, and the layout definition uses `set` and `binding` instead of `location`. This is because `t_diffuse` and `s_diffuse` are what's known as *uniforms*. We won't go too deep into what a uniform is, until we talk about uniform buffers in the [cameras section](/beginner/tutorial6-uniforms/).
For now, all we need to know is that `set = 0` corresponds to the 1st parameter in `set_bind_group()` and `binding = 0` relates the the `binding` specified when we create the `BindGroupLayout` and `BindGroup`.
That's weird, our tree is upside down! This is because wgpu's world coordinates have the y-axis pointing up, while texture coordinates have the y-axis pointing down. In other words, (0, 0) in texture coordinates coresponds to the top-left of the image, while (1, 1) is the bottom right.
2. We're returning a `CommandBuffer` with our texture. This means we could load multiple textures at the same time, and then submit all there command buffers at once.
