Thursday, 2 May 2013

My camera's sensor's linear capture set against the camera's processed jpegs

Like the course said, when I compared the 'linear' representation of what the sensor captured to the processed jpeg side to side, I could see how the histograms (the graphs to the right of each image) differed. The histogram representing the camera's sensor's linear capture (left in Figure 1) showed the pixels with values were congregated on the left, which meant the image was darker and possessed less dynamic range. None of the pixels with values for this image were on the right.

Figure 1 - linear capture on left, processed jpeg on right
   In contrast, there was a much more even distribution of pixels with values in the processed jpeg image (right in Figure 1). There was incidentally quite a lot of levels of tones on the right but they only gathered there because of the large amount of brighter light in the sky. More importantly for this exercise, there weren't many levels of tones on the left (the shadow areas) because the jpeg had already procesed  the image captured by the sensor to produce a result more akin to what my eyes had seen. So overall, I observed the histogram had moved from the left (with the camera's sensor's linear capture representation) on the left of Figure 1, to the right (the camera's processed jpeg) on the right of Figure 1. This was because of the gamma correction curve applied to the camera's sensor's linear capture.

Figure 2 - processed jpeg on the right, reprocessed linear capture on the right

Figure 3 - 100% view of shadow area
Figure 4 - histograms for both the processed jpeg and the reprocessed linear capture
   When I reopened the representation of the camera's sensor's linear capture to see how it would cope with being processed to form an image that as closely as possible resembled the processed jpeg I found the results to be quite insightful. Firstly, there was visible noise in the reprocessed linear capture even at monitor size viewing compared to the processed jpeg (as in Figure 2). This of course increased drastically at 100% viewing as can be seen in Figure 3. Secondly, the colours were a lot less saturated than the camera's processed jpeg. Finally, I found when looking closely at the two histograms in Figure 4 there was quite severe fragmenting in the reprocessed linear capture's histogram compared to the 'smooth' histogram of the camera's processed jpeg. This was a clue as to why the shadow noise was more and the saturation was less in the reprocessed linear capture. The reason I summised was that the decreased amount of information in the shadow areas had led to the reprocess of the camera's linear capture not making such a smooth transition between dark and midtone valued areas.

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