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Forum: Photographic Technique 09-24-2017, 04:27 PM  
Hand held Pixel-Shifting...
Posted By photoptimist
Replies: 32
Views: 5,005
Resolution is the ability to resolve two objects as separate things rather than one blob. If I create a scene with two red dots (or two thin red lines) on a black background and measure how close they can be before a camera can no longer resolve them, then I'd see that a both the Foveon and pixelshift camera have twice the resolution as the normal Bayer filter camera becuase they make 4X the number of red channel measurements. With the Bayer camera, the red dots or lines must be at least 4 pixels apart so that in a given RGRGRGRGR row of pixels, there is a dark R pixel between two bright R pixels. With pixelshift and Foveon, the camera can resolve lines or dots with only a 2 pixel spacing.

The model by which the demosaicer uses surrounding pixels to estimate color for a patch and separately estimate light-and-dark variations makes dangerous assumptions about the nature of the object. The demosaicer assumes color does not vary much over short distances which is to say a Bayer camera cannot resolve high-resolution variation in color. It's why pictures of fine black-and-white objects often have weird color artifacts. It also causes problems with star color in astrophotography depending upon how the bright point of the star falls more on one color or another.

But we can agree about handheld pixelshift and talking over coffee! :)
Forum: Photographic Technique 09-24-2017, 01:44 PM  
Hand held Pixel-Shifting...
Posted By photoptimist
Replies: 32
Views: 5,005
I'm assuming a frame-to-frame statistical distribution of motion with a mean of at least a couple of pixels, a standard deviation of at least a couple of pixels, and random angle. It doesn't take much motion for the chance of an R, G, or B pixel landing on a given spot to become almost totally random. Of course, there's always some chance of really weird results with interactions between hand-held motions (which probably have pretty strong autcorrelation in velocity and direction) and the geometry of the Bayer filter (which has strong structure for some angles and frame-to-frame spacings).

The most sophisticated stacking algorithms used in astrophotography and remote sensing really do model each pixel as a rectangular sampling area that intersects with other rectangular sampling areas and the algorithm can compute the most likely sub-pixel resolution structure of the scene that would give rise to all the data. But it takes a lot of frames, a lot of computer power, and a sophisticated calibration of the lens and sensor. And it can never guarantee uniformly high resolution across the frame because of interactions in all the periodic sampling functions.

Pixel shift needs only four frames and always improves a big boost to resolution as long as the lens is good enough, the scene is stationary, and the lighting levels are stable.
Forum: Photographic Technique 09-24-2017, 08:02 AM  
Hand held Pixel-Shifting...
Posted By photoptimist
Replies: 32
Views: 5,005
If you take a picture of a red object with a K-1 and measure the resolution in line pairs per mm, you'll find that the K-1 acts like a 9 MPix camera because it samples red in only 9 million places on the scene.

If you take a picture of a red object with a K-1 in pixel shift mode and measure the resolution in line pairs per mm, you'll find that the K-1 PS acts like a 36 MPix camera because it then samples red in a total of 36 million places on the scene.

Pixel shift most assuredly boosts resolution in line pairs per mm, especially in red and blue but also in green, top. (see Pentax K-3 II Review - Pixel Shift Resolution mode for examples)

The color depth of a Bayer filter image is actually better than you think because the demosaicer averages together neighboring pixels. But the resolution of the shape of color object is much worse with Bayer than with pixelshift.

---------- Post added 09-24-17 at 09:30 AM ----------

Pedantic is good! It's just another term for precise and accurate! Yes, pixelshift is not like most other stacking algorithms for the reasons that you state. Pixelshift is more of a careful disassembly of the 4 carefully captured frames with reassembly into a final true full resolution full color image.

The percentage figures in the last paragraph come from simple statistical calculations of the chance that a given point in the scene (i.e., a given pixel in the output image) was ever visited by all three categories of color sensor. For example if one shoots four hand-held frames will little motions between each frame, it's entirely possible that by chance a certain pixel in the scene was only sampled by green camera pixels. The probability of that occurrence is (1/2)^4 or 1/16. One can use basic probability math to estimate the chance that a place in the scene is never sampled in red, or blue, or green, or whatever. And if a place was not sampled by all colors, the resolution will be lower because the stacker was forced to interpolate.

A more careful analysis of this kind of superresolution stacking of images under camera motion will reveal very complex pixel-by-pixel patterns of higher and lower resolution in which some pixels were well sampled by all three colors and others were not. There's even a tiny chance that the superresolution stack is no better than a single frame because by chance the Bayer filter colors lined up on every shot.
Forum: Photographic Technique 09-23-2017, 06:17 PM  
Hand held Pixel-Shifting...
Posted By photoptimist
Replies: 32
Views: 5,005
Pixel shift most certainly does increase resolution significantly.

It may be true that the Bayer filter means the image pixels have a color pattern such as:

R G R G
G B G B
R G R G
G B G B

That filter design means that resolution in red has 1/4 the pixels of the full array (1/2 the linear resolution of details), the same with blue, and green has 50% of the pixels (about 70% the resolution of the full array). In normal mode, the K-1 gets a 9 MPix red image, a 9 Mpix blue image, and an 18 MPix green image which it interpolates to create a 36 MPix color image.


But that's only in one frame. In pixel shift, the second frame shifts the sensor array one pixel to the left to get color array pattern covering the scene of:

G R G R
B G B G
G R G R
B G B G

Then it shifts one pixel up to get:

B G B G
G R G R
B G B G
G R G R

Finally, it shift one pixel back to the right to get:

G B G B
R G R G
G B G B
R G R G

Those four images are stored in the massive RAW file that is 4X the size of a regular one.

If you look carefully, every location in the scene was measured once in red, once in blue, and twice in green.

The image developer stacks the four images to create a true 36 MPix x 3 color image with the green image have a noise-reduced double-sampling. The result is BOTH much higher resolution AND better color depth.


Yes, you could just grab a hand-held stack of images and use super-resolution techniques but you need a lot more than 4 images to get resolution results as good as pixel shift because the super-resolution stacking technique relies on a lot of luck that every bit of the scene was visited by sensor pixels of all the three colors. With only 4 hand-held frames stacked, about 32% of pixel locations will be missing the red pixel data, about 32% will be missing blue pixel data, but about 6% will be missing green pixel data. Even with 8 frames stacked, about 10% of pixel locations will be missing the red pixel data, about 10% will be missing blue pixel data, but almost all (99.6%) will have green pixel data. It will be good, but not as good as pixel shift which ensures ever spot gets measured.
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