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WarpBlt
As we began doing more with general rotation and scaling of images, we found ourselves dissatisfied with the slow speed of non-integer scaling and image rotations by angles other than multiples of 90 degrees. To address this problem in a simple manner, we added a "warp drive" to BitBlt. WarpBlt takes as input a quadrilateral specifying the traversal of the source image corresponding to BitBlt's normal rectangular destination. If the quadrilateral is larger than the destination rectangle, sampling occurs and the image is reduced. If the quadrilateral is smaller than the destination, then interpolation occurs and the image is expanded. If the quadrilateral is a rotated rectangle, then the image is correspondingly rotated. If the source quadrilateral is not rectangular, then the transformation will be correspondingly distorted.
Once we started playing with arbitrarily rotated and scaled images, we began to wish that the results of this crude warp were not so jagged. This led to support for over sampling and smoothing in the warp drive, which does a reasonable job of anti-aliasing in many cases. The approach is to average a number of pixels around a given source coordinate. Averaging colors is not a simple matter with the table-based colors of 8 bits or less. The approach we used is to map from the source color space to RGB colors, average the samples in RGB space, and map the final result back to the nearest indexed color via the normal depth-reducing color map.
As with the sound synthesis work, WarpBlt is completely described in Smalltalk, then translated into C to deliver performance appropriate to interactive graphics.
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