An HDR photograph can start with one exposure. That statement needs a qualification: no software can create scene information that the sensor never recorded. The opportunity is to develop the information already present in a RAW file with more luminance headroom, instead of compressing it all into an SDR rendering.
Capture and display are different problems
Exposure bracketing records the same scene at several exposures and merges them to exceed the limits of a single capture. It remains useful when scene contrast outstrips the sensor's range: a dark interior with a sunlit window is a familiar example.
HDR display solves another problem. A recent RAW may hold highlight and shadow information that a conventional SDR JPEG must compress. On an HDR display, some of that information can appear as genuinely higher luminance: brighter reflections, while midtones stay natural. Greg Benz provides a useful introduction to RAW files and HDR displays.
Acutance Studio works with this second opportunity. It develops separate SDR and HDR renditions from a single digital negative. It does not require three or five frames, or the alignment of a bracketed sequence. Its Ultra HDR JPEG export carries both renditions in one file by means of a gain map.
Where to look for the difference
Look at areas that were light sources or reflections in the scene: sun through leaves, droplets on glass, a lamp on a wet street. They can gain sparkle without pushing the entire photograph toward glaring white. Good HDR processing need not lift every shadow or remove the local contrast that gives objects their shape.
The comparison section uses paired SDR and Ultra HDR exports supplied with the suite. On an SDR screen you see only the compatible base of the HDR JPEG. To see the actual luminance difference, you need an enabled HDR display and a viewer that reads the gain map.
Respect the limit of the exposure
If an area of the RAW is saturated in every channel, a slider cannot recreate its original texture. If a shadow contains little beyond sensor noise, brightening it will not reveal clean detail. In those conditions, a more careful exposure or bracketing remains useful. The single-RAW workflow is about making full use of one capture when that capture contains the needed information.
Check the result by toggling SDR and HDR on the same monitor in Acutance Photo-View (H), and inspect the export on an SDR display too. The photograph should work in both contexts.