It covers HDR creation through the merging of multiple exposures (using hugin’s align_image_stack functionality if the images in the stack are not perfectly aligned). And it does tonemapping, offering a choice of eight tone mapping operators (TMO). This is what I like most about Qtpfsgi: the variety of processing options that it offers. Each TMO has its own strenght and weaknesses, and thus its own fields of application. Try them all, you will find your favorites for the different shooting situations!
We’ll be looking in detail at the individual TMOs in the coming articles.
Common to all of them is that
- we need an HDR image to start with
- the image can be subject to a pre tone mapping gamma adjustment
Image to start with
Qtpfsgui can be used to compose HDR images from individual exposures. Or it can load the most common HDR file formats. Last but not least, it also load RAW images straight out of the camera. Granted, RAW images have more dynamic range than JPEG images, but in most cases they are still far away from true HDR. Nevertheless, applying TMO’s to RAW images can yield some stunning results. Dare to experiment!
Also it is important to note that the appropriate TMO settings and the results change depending on the type of image processed. Dare to experiment with all sorts of images and available TMOs!
Pre Tone Mapping Gamma Adjustment
Gamma correction is the application of an exponential function to compress (gamma <1) or expand (gamma>1) the image’s dynamic range. A gamma of 1 leaves it unchanged. Gamma <1 compresses the range, resulting in generally higher contrast and darker images. Gamma > 1 expands the range, resulting in generally brighter and lower contrast images. You will find out during the processing with the selected TMO whether applying a pre tone mapping gamma adjustment improves the image.
Last Warnings Before We Go
Tone mapping is a highly subjective topic. There is no right or wrong – follow your inspiration and develop your own style. Nevertheless, there are a few pitfalls:
Noise: If the source images are affected by noise, it is likely to be amplified by the TMOs. Try to shoot at low ISO, or filter noise in the input image before merging to HDR.
Settings: Don’t expect the settings for one image to do the same magic on a completely different image. Settings are case by case. With time you will develop a feeling for each situation.
Saturation: A lot of tone mapped images are over saturated. Keep an eye on it.
Post processing: You will often want to load your tone mapped image and use simple adjustment layers such as levels and curves in your favorite image editor. The goal of the tone mapping process is to reveal the detail, not to finish the image.