As you can imagine, there are many uses for such functionality. Following the calculation, an output raster is produced that contains a new value for each cell (Figure 8.4). The numbers within the aligned cells of the input grids can undergo any user-specified mathematical transformation. The mathematical raster overlay is the most common overlay method. Exploring Geographic Information Systems. With this in mind, there are several different methodologies for performing a raster overlay (Chrisman 2002).Chrisman, N. If these assumptions are violated, the analysis will either fail or the resulting output layer will be flawed. Despite their simplicity, it is important to ensure that all overlain rasters are coregistered (i.e., spatially aligned), cover identical areas, and maintain equal resolution (i.e., cell size). Geographical Information Systems for Natural Resources Assessment. Raster overlays are relatively simple compared to their vector counterparts and require much less computational power (Burroughs 1983).Burroughs, P. Figure 8.3 Clipping a Raster to a Vector Polygon Layer The raster clip process results in a single raster that is identical to the input raster but shares the extent of the polygon clip layer. Here, the input raster is overlain by a vector polygon clip layer. Figure 8.2 Raster Buffer around a Target Cell(s)Ī raster dataset can also be clipped similar to a vector dataset (Figure 8.3). These cells could also be further classified to represent multiple ring buffers by including values of “3,” “4,” “5,” and so forth, to represent concentric distances around the target cell(s). Most geographic information system (GIS) programs calculate raster buffers by creating a grid of distance values from the center of the target cell(s) to the center of the neighboring cells and then reclassifying those distances such that a “1” represents those cells composing the original target, a “2” represents those cells within the user-defined buffer area, and a “0” represents those cells outside of the target and buffer areas. Figure 8.1 Raster ReclassificationĪs described in Figure 8.2). In addition, these reclassified layers are often used as inputs in secondary analyses, such as those discussed later in this section. This simplification allows for fewer unique values and cheaper storage requirements. These values could be simplified by aggregating each pixel value in a few discrete classes (i.e., 0–100 = “1,” 101–200 = “2,” 201–300 = “3,” etc.). For example, an elevation grid commonly contains a different value for nearly every cell within its extent. Reclassification is basically the single layer process of assigning a new class or range value to all pixels in the dataset based on their original values (Figure 8.1. Reclassifying, or recoding, a dataset is commonly one of the first steps undertaken during raster analysis. The raster data is snapped to integer locations that are then culled and clipped (to draw the minimum number of pixels), and per-pixel attributes are interpolated (from per-vertex attributes) before being passed to a pixel shader.\) Rasterization rules define how vector data is mapped into raster data. This section describes setting the viewport, the scissors rectangle, the rasterizer state, and multi-sampling. Getting Started with the Rasterizer Stage On hardware that implements hierarchical Z-buffer optimizations, you may enable preloading the z-buffer by setting the pixel shader stage to NULL while enabling depth and stencil testing. There is also a complete description of the rasterization rules. While disabled, rasterization-related pipeline counters will not update. You may disable rasterization by telling the pipeline there is no pixel shader (set the pixel shader stage to NULL with ID3D11DeviceContext::PSSetShader), and disabling depth and stencil testing (set DepthEnable and StencilEnable to FALSE in D3D11_DEPTH_STENCIL_DESC). In this coordinate space the X axis points right, Y points up and Z points away from camera. Vertices (x,y,z,w), coming into the rasterizer stage are assumed to be in homogeneous clip-space. While using a pixel shader is optional, the rasterizer stage always performs clipping, a perspective divide to transform the points into homogeneous space, and maps the vertices to the viewport. Rasterization includes clipping vertices to the view frustum, performing a divide by z to provide perspective, mapping primitives to a 2D viewport, and determining how to invoke the pixel shader. The rasterization stage converts vector information (composed of shapes or primitives) into a raster image (composed of pixels) for the purpose of displaying real-time 3D graphics.ĭuring rasterization, each primitive is converted into pixels, while interpolating per-vertex values across each primitive.
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