Image-based visual discomfort analysis has strong potential to detect glare in order to predict occupant satisfaction with a space. However, in order to design buildings with occupant visual comfort in mind, architects need the ability to predict glare through renderings, rather than photographs. In this paper, we compare glare measurements from high dynamic range photographs and computer renderings of the same daylit space under 240 clear and 38 cloudy sky conditions in order to assess how accurately architects can predict glare though simulation. We use two ray tracing engines, Radianceand Accelerad, on scenes illuminated by a mathematically defined sky model or by recorded outdoor luminance levels from photographs. Depending on the scene, these methods predict daylight glare probability levels due to bright sources with between 93% and 99% accuracy and discomfort glare due to contrast with between 71% and 99% accuracy. Glare predictions are more sensitive to modeling and measurement accuracy than to simulation parameters, so accurate results can come from relatively fast simulations. Furthermore, with graphics acceleration, we are able to produce accurate glare predictions within minutes using Accelerad, achieving a speedup over Radiance of between 16 and 44 times.