Autodesk Maya 2017 is a powerful, integrated 3D modeling, animation, visual effects, and rendering software developed by Autodesk Inc. You can use the UV Toolkit's Measure tool to keep track of pixel distances.Welcome to the world of Autodesk Maya 2017. Thus, when doing the layout you need to make sure that you have at least 16px distance between shells and 8px distance to the UV map border. In this case, the spacing needs to be 4px between shells at 512px, then 8px at 1024px, then 16px at 2048px. For example: If you have an asset with 3x LOD steps and an original texture size of 2048px, then on the smallest mipmap level the texture is only 512px.
If LOD models are being used, then every LOD/Mipmap step requires double the shell spacing. If the texture map is further reduced in size by the engine you need to increase this padding. However, that only applies to the final version of the texture. In general, it's good practice to keep at least 2 pixels around all UV shells so that there is a 2px margin to the texture map border, and a 4px margin between UV shells. Texture bleeding: Texture bleeding is when the color information inside one UV shell bleeds into another UV shell due to texture filtering. There are a few things to note when it comes to shell spacing: Shell spacing refers to the amount of space between UV shells (also known as shell padding). When doing environment art, you are strongly advised to use texture atlases. 
Loading textures into the graphics memory is slowed down by the following factors: How many channels there are in the texture (RGBA is 4x more expensive than grayscale), how many pixels there are in the texture, and how many texture maps your asset uses. This way, if you end up with too little or too much UV space left, you can simply select your entire UV layout and scale it. Work from one corner towards the opposite one, while trying to keep the layout ratio intact. Also consider going for another ratio than 1:1, such as 2:1 or even 4:1, as the shape of the texture map does not affect the texture processing at all. Divide and conquer: Start laying out UV shells by placing the largest and most oddly-shaped shells into the UV range (0 to 1) first. To make symmetry mapping easier, cut your mesh in half before performing a layout. Sometimes you can even add extra geometry around this logo in order to cut out that particular part of the shell, allowing everything else from the large shells to be stacked. If you have some form of text or logo on one side of the model that you don't want mirrored to the other, consider mirroring the entire shell except the area with the logo. However, when doing so be careful to consider what kind of ambient occlusion shadows the affected shells will receive (if you use AO). Symmetry mapping: When working on symmetrical meshes, you can stack shells on top of each other that are mirrored over a plane, or even radially, so that they reference the same portion of the texture. For example if you are working on a FPS weapon with iron sights then the scope part should have the highest density, and the right and front side of the gun the lowest. Start by setting a uniform texel density to all UV shells and then scale up or down according to those factors. Consider what parts of your model are going to be visible to the camera, how often, and at what distances. 
Unless you are mapping for technical models or VFX assets, you need to think about optimizations that will enhance performance while reducing memory usage. See the Optimization section below for more details. Additionally for mobile graphics, consider pushing the UVs of a shell into a line segment if you only need information in one direction (like a gradient) or even a point (if you only need a color). Do not be afraid of straightening border shells so that you can pack shells tighter together so that they fit within the 0 to +1 UV space. Furthermore, shells are oriented straight along U/V, and those with odd shapes are cut up into smaller parts. Every shell that can be mirrored (along a plane or radially) is often stacked on top of each other. The Texel density will vary a lot and stretching and pinching will be created on purpose where it's needed. Heavily optimized UV maps are most common when working with mobile/low-poly graphics, where you have heavy limitations on your assets, but also for current gen graphics since loading texture maps into the graphics memory is heavy work.
The end-goal is to have as much coverage on a map that is as small as possible. One extreme is a heavily optimized UV map, which is used exclusively for real-time graphics.
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