Drag the handles to change the quad's bounds. Click on the icons at the bottom-right to change texture. Press the Z key to show/hide the variable overlay. You can also supply a custom texture of your own - to do this, click on the "?" button for directions, then click on the "Custom Texture" button to view it. Link to the tool for converting images to use in this project: http://www.tiberiumfusion.com/external/scratch/texturemapping/
Performance isn't terrible, but 20ms (18ms with Phosphorus) for 625 pixels is fairly limiting to some project plans I have. Enabling Auto Pen Size generally doubles the render time. For some time now, I've wanted to add textures to my SNAA 3D engine. Efficiently mapping vertices to uv coords and rendering texture partials in triangles is the ultimate goal, and certainly no simple one at that. Often times, solving a problem is easier if a simpler version is solved first, which is the cause of this project. Mapping entire square textures to quads is a good stepping stone, I think, to the end goal.