I decided to jot down some notes made from some algorithms that I previously worked on. These are mostly high level descriptions providing some intuition on how things work.

**Exposure Fusion:**

- Is a method of producing a high quality LDR (low dynamic range) image from a series of LDR images taken at different exposures (bracketed exposure)
- It does not require a conversion step to HDR, e.g. like tone mapping that is needed for a display device to show the HDR image properly
- Global operators for tone mapping – Apply some scaling to the entire image to convert to LDR
- Local operators – Do different things in different regions of the image to preserve detail
- Exposure Fusion uses 3 quality measures — namely ‘contrast’, ‘saturation’ and ‘well-exposedness’
- For contrast – Apply Laplacian Filter
- For saturation – Compute the standard deviation within R,G,B channels
- For well-exposedness – We want to keep pixels which are neither over-exposed (close to 1) nor under-exposed (close to 0) , so we want to keep values close to 0.5 using a Gaussian curve

- We combine the above info into a weight map using multiplication
- If we just apply the weight map to each of the images in the bracketed exposure there will very sharp transitions and undesirable halos etc.
- To counter that, we blend images at multiple resolutions using a pyramid method
- First create a Laplacian pyramid of the images
- Then create a Gaussian pyramid of the weight maps
- Combine at each level
- Finally you end up with one pyramid – Now repeat the sequence of ‘Up sample, blur, combine’ until you get the final image

**Panorama Stitching with Ransac Homography:**

- Use SURF/SIFT feature detector to get the keypoints for both the images
- Use Flann Matcher to get the matches
- Find the min and max distance between keypoints
- Take matches whose distance is less than some threshold (3 * min distance)
- Find homography using these set of matched points
- A matrix H which transforms vector a to vector b (aH = b, H = b * inverse(a))
- vector a & b contain the 4 points respectively
- use singular value decomposition to solve for this matrix
- Use this matrix to find out how many good matched points are transformed successfully, check ‘inliers’ that are below a threshold value in terms of distance
- Find the matrix that produces the best set of inliers, that is the homography matrix to use
- warp the image using this matrix

An eigenvector v of a linear transformation T is a non-zero vector that, when T is applied to it, does not change direction

**Sobel Filter:**

- Edge detection filter that accounts for gradients in both the x and y directions.
- 2 filters are used, namely:

**Laplace Filter:**

**Laplacian of Gaussian:**

Laplace filter is extremely sensitive to noise, so smoothening it with a Gaussian filter first is good.

**Reinhard Tonemapping Operator:**

- Summation of log luminance and average
- operator = L ( 1 + L) — small luminance is scaled by 1 and large ones are penalized.
- Has 2 operators – global and local

**Bilateral Filter:**

- Using a simple Gaussian filter causes halos due to large intensity differences.
- To counter this, we applied a penalty on the intensity difference.
- So we start with a spatial Guassian, then a penalty Gaussian on the intensity difference.
- This is done by creating patches of the image around a certain pixel.
- The spatial Gaussian is taken of the patch, and the penalty Gaussian is calculated using the intensity difference between the rest of the patch and the current pixel.

**Set of operations:**

- Input is the RGB values of radiance, possibly in radiance format
- Compute the intensity (I) at each pixel. E.g. use a weighted average: 1/61*(R*20 + G*40 + B)
- Compute the chrominance channels: R/I, G/I, B/I
- Compute the log of the intensity: L = log2(I)
- Apply bilateral filter: B = bf(L)
- Compute the detail layer: D = L-B – this should contain the edge information
- Apply an offset and a scale to the base: B’ = (B-o)*s
- Reconstruct the log intensity: O = exp(B’+D)
- Put the colors back: R’,G’,B’ = O*(R/I, G/I, B/I)
- Apply a gamma compression

**Scale Invariant Feature Transform (SIFT):**

- Construct a scale space
- each octave is for one size and then many levels of blur within each octave

- LoG approximation
- within octave subtract pairs to get LoG

- Find keypoints
- find maxima/minima
- find subpixel maxima/minima
- check if point is min or max among the neighbors around and on top and bottom layer in scale space

- Filter keypoints
- Keypoint orientation
- flat both gradients are small, edge – one is big other is small, corner – both are big
- gradient magnitudes and orientation – using a histogram – 360 deg into 36 bins

- Feature generation
- 16×16 window around a keypoint, 4*4 squares within this – calc grad and ori magnitudes within each 4×4 and add to a histogram

**Flann Matching:**

Fast library for Nearest neighbor search – k nearest neighbors

Advertisements