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how to implement grayscale rendering in OpenGL?
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Anyone know how to convert to grayscale, below is some skeleton code i need to use to do so. Specifically, convert "before" to grayscale,
apply the Sobel edge detection convolution filter, and store the
result in "after". before must be non-empty.
template <typename color_depth> void
edge_detect(gfx::image<color_depth>& after,
const gfx::image<color_depth>& before) {
// Check arguments.
assert(!before.empty());
// TODO: replace this function body with working code. Make sure
// to delete this comment.
// Hint: Use the grayscale(...) and extend_edges(...) filters to
// prepare for the Sobel convolution. Then compute the Sobel
// operator one pixel at a time. Finally use crop_extended_edges
// to un-do the earlier extend_edges.
}
This looks to be a homework question so I won't give a full implementation. I also can't tell if you want to convert to greyscale on the CPU or in a shader. Regardless of where you perform the conversion the formulas are the same.
There is no definitive method for converting to greyscale since since you're discarding information and whether the end results look correct is entirely subjective. Below are some common methods for converting from RGB to greyscale:
A naive approach is to find the colour channel with the highest value and just use that.
grey = max(colour.r, max(colour.g, colour.b));
The naive approach suffers in that certain areas of your image will lose detail completely if they contain none of the colour with the highest value. To prevent this we can use a simple average of all the colour components.
grey = (colour.r + colour.g + colour.b) / 3.0;
A 'better' method is to use the luma value. The human eye perceives some colour wavelengths better than others. So if we give more weight to those colours we produce a more plausible greyscale.
grey = dot_product(colour, vec3(0.299, 0.587, 0.114));
Yet another method is to 'desaturate' the image. This involves first converting from the RGB colour space to HSL. Then reducing the saturation to zero.
Related
Matlab offers the ability to set colour limits for the current axis using CAXIS. OpenCV has applyColorMap which can be used to highlight differences in pixel intensity in a greyscale image which I believe maps pixel from 0 - 255.
I am new to Matlab/Image-processing and have been asked to port a simple program from MatLab which uses the CAXIS function to change the "brightness" of a colour map. I have no experience in Matlab but it appears that they use this function to "lower" the intensity requirements needed for pixels to be mapped to a more intense colour on the map
i.e. Colour map using "JET"
When brightness = 1, red = 255
When brightness = 10, red >= 25
The matlab program allows 16bit images to be read in and displayed which obviouly gives higher pixel values whereas everything i've read and done indicates OpenCV only supports 8 bit images (for colour maps)
Therefore my question is, is it possible to provide similar functionality in OpenCV? How do you set the axis limit for a colourmap/how do you scale the colour map lookup table so that "less" intense pixels are scaled to the more intense regions?
A similar question was asked with a reply stating the array needs to be "normalised" but unfortunately I don't quite know how to achieve this and can't reply to the answer as i don't have enough rep!
I have gone ahead and used cv::normalize to set the max value in the array to be maxPixelValue/brightness but that doesn't work at all.
I have also experimented and tried converting my 16bit image into a CV_8UC1 with a scale factor to no avail. Any help would be greatly appreciated!
In my opinion you can use cv::normalize to "crop" values in the source picture to the corresponding ones in color map you are interested in. Say you want your image to be mapped to the blue-ish region of Jet colormap then you should do something like:
int minVal = 0, maxVal = 80;
cv::normalize(src,dst, minVal, maxVal, cv::NORM_MINMAX);
If you plan to apply some kind of custom map it's fairly easy for 1-or3-channel 8-bit image, you only need to create LUT with 255 values (with proper number of channels) and apply it using cv::LUT, more about it in this blog, also see the dosc about LUT
If the image you are working is of different depth, 16-bit or even floating point data I guess all you need to do is write a function like:
template<class T>
T customColorMapper(T input_pixel)
{
T output_pixel = 0;
// do something with output_pixel basing on intput_pixel
return output_pixel;
}
and apply it to each source image pixel like:
cv::Mat dst_image = src_image.clone(); //copy data
dst_image.forEach<TYPE>([](TYPE& input_pixel, const int* pos_row_col) -> void {
input_pixel = customColorMapper<TYPE>(input_pixel);
});
of course TYPE need to be a valid type. Maybe specialized version of this function taking cv::Scalar or cv::Vec3-something would be nice if you need to work with multiple channels.
Hope this helps!
I managed to replicate the MATLAB behaviour but had to resort to manually iterating over each pixel and setting the value to the maximum value for the image depth or scaling the value where needed.
my code looked something like this
cv::minMaxLoc(dst, &min, &max);
double axisThreshold = floor(max / contrastLevel);
for (int i = 0; i < dst.rows; i++)
{
for (int j = 0; j < dst.cols; j++)
{
short pixel = dst.at<short>(i, j);
if (pixel >= axisThreshold)
{
pixel = USHRT_MAX;
}
else
{
pixel *= (USHRT_MAX / axisThreshold);
}
dst.at<short>(i, j) = cv::saturate_cast<short>(pixel);
}
}
In my example I had a slider which adjusted the contrast/brightness (we called it contrast, the original implementation called it brightness).
When the contrast/brightness was changed, the program would retrieve the maximum pixel value and then compute the axis limit by doing
calculatedThreshold = Max pixel value / contrast
Each pixel more than the threshold gets set to MAX, each pixel lower than the threshold gets multiplied by a scale factor calculated by
scale = MAX Pixel Value / calculatedThreshold.
TBH i can't say I fully understand the maths behind it. I just used trial and error until it worked; any help in that department would be appreciated HOWEVER it seems to do what i want to!
My understanding of the initial matlab implementation and the terminology "brightness" is in fact their attempt to scale the colourmap so that the "brighter" the image, the less intense each pixel had to be to map to a particular colour in the colourmap.
Since applycolourmap only works on 8 bit images, when the brightness increases and the colourmap axis values decrease, we need to ensure the values of the pixels scale accordingly so that they now match up with the "higher" intensity values in the map.
I have seen numerous OPENCV tutorials which use this approach to changing the contrast/brightness but they often promote the use of optimised convertTo (especially if you're trying to use the GPU). However as far as I can see, convertTo applies the aplha/beta values uniformly and not on a pixel by pixel basis therefore I can't use that approach.
I will update this question If i found more suitable OPENCV functions to achieve what I want.
I want to take a video and create a binary from it, I want it so that if the pixel is within a certain range it will be included within the binary. In other words I want an upper and lower bound like in the inRange() function as opposed to a simple cutoff point like in the threshold() function.
I also want to use adaptive thresholding to account for differences in lighting in my video. Is there a way to do this? I know there is inRange() that does the former and adaptiveThreshold() that does the latter, but I don't know if there is a way to do both.
Apply adaptiveThreshold() to the whole original image, then apply inRange() to the original image and use the result of inRange() as a mask:
adaptiveThreshold(original_image, dst_image ... );
inRange(original_image, minArray, maxArray, mask);
Mat output = dst_image.mul(mask);
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Please explain as to what happens to an image when we use histeq function in MATLAB? A mathematical explanation would be really helpful.
Histogram equalization seeks to flatten your image histogram. Basically, it models the image as a probability density function (or in simpler terms, a histogram where you normalize each entry by the total number of pixels in the image) and tries to ensure that the probability for a pixel to take on a particular intensity is equiprobable (with equal probability).
The premise behind histogram equalization is for images that have poor contrast. Images that look like they're too dark, or if they're too washed out, or if they're too bright are good candidates for you to apply histogram equalization. If you plot the histogram, the spread of the pixels is limited to a very narrow range. By doing histogram equalization, the histogram will thus flatten and give you a better contrast image. The effect of this with the histogram is that it stretches the dynamic range of your histogram.
In terms of the mathematical definition, I won't bore you with the details and I would love to have some LaTeX to do it here, but it isn't supported. As such, I defer you to this link that explains it in more detail: http://www.math.uci.edu/icamp/courses/math77c/demos/hist_eq.pdf
However, the final equation that you get for performing histogram equalization is essentially a 1-to-1 mapping. For each pixel in your image, you extract its intensity, then run it through this function. It then gives you an output intensity to be placed in your output image.
Supposing that p_i is the probability that you would encounter a pixel with intensity i in your image (take the histogram bin count for pixel intensity i and divide by the total number of pixels in your image). Given that you have L intensities in your image, the output intensity at this location given the intensity of i is dictated as:
g_i = floor( (L-1) * sum_{n=0}^{i} p_i )
You add up all of the probabilities from pixel intensity 0, then 1, then 2, all the way up to intensity i. This is familiarly known as the Cumulative Distribution Function.
MATLAB essentially performs histogram equalization using this approach. However, if you want to implement this yourself, it's actually pretty simple. Assume that you have an input image im that is of an unsigned 8-bit integer type.
function [out] = hist_eq(im, L)
if (~exist(L, 'var'))
L = 256;
end
h = imhist(im) / numel(im);
cdf = cumsum(h);
out = (L-1)*cdf(double(im)+1);
out = uint8(out);
This function takes in an image that is assumed to be unsigned 8-bit integer. You can optionally specify the number of levels for the output. Usually, L = 256 for an 8-bit image and so if you omit the second parameter, L would be assumed as such. The first line computes the probabilities. The next line computes the Cumulative Distribution Function (CDF). The next two lines after compute input/output using histogram equalization, and then convert back to unsigned 8-bit integer. Note that the uint8 casting implicitly performs the floor operation for us. You'll need to take note that we have to add an offset of 1 when accessing the CDF. The reason why is because MATLAB starts indexing at 1, while the intensities in your image start at 0.
The MATLAB command histeq pretty much does the same thing, except that if you call histeq(im), it assumes that you have 32 intensities in your image. Therefore, you can override the histeq function by specifying an additional parameter that specifies how many intensity values are seen in the image just like what we did above. As such, you would do histeq(im, 256);. Calling this in MATLAB, and using the function I wrote above should give you identical results.
As a bit of an exercise, let's use an image that is part of the MATLAB distribution called pout.tif. Let's also show its histogram.
im = imread('pout.tif');
figure;
subplot(2,1,1);
imshow(im);
subplot(2,1,2);
imhist(im);
As you can see, the image has poor contrast because most of the intensity values fit in a narrow range. Histogram equalization will flatten the image and thus increase the contrast of the image. As such, try doing this:
out = histeq(im, 256); %//or you can use my function: out = hist_eq(im);
figure;
subplot(2,1,1);
imshow(out);
subplot(2,1,2);
imhist(out);
This is what we get:
As you can see the contrast is better. Darker pixels tend to move towards the darker end, while lighter pixels get pushed towards the lighter end. Successful result I think! Bear in mind that not all images will give you a good result when you try and do histogram equalization. Image processing is mostly a trial and error thing, and so you put a mishmash of different techniques together until you get a good result.
This should hopefully get you started. Good luck!
I am attempting to write a piece of code that is suppose to map data to RGB values, and one of the types of visualizations I am attempting to use is a diverging color map.
I am not exactly sure what the best way is to go about applying the colors. The current algorithm I am using is:
//F is the data point being checked
if(F <= .5){
RGB[0] = F*510;
RGB[1] = F*510;
RGB[2] = F*254 + 128;
}else{
RGB[0] = 255 - (F-.5)*254;
RGB[1] = 255 - (F-.5)*510;
RGB[2] = 255 - (F-.5)*510;
}
Where the key points for the curve are:
F=0: (0,0,128)
F=0.5: (255,255,255)
F=1: (128, 0, 0)
Are there any suggested algorithms out there for use instead of this, or is this hacked together piecewise function alright?
This is the image generated by this current algorithm.
I think you should use a bar to test your function as it would be easier to see the transition 'speed' in linear data.
Here is a really good article for using the diverging colour maps: http://www.sandia.gov/~kmorel/documents/ColorMaps/
It describes the mathematics behind it. I know it seems an overkill to go through Lab and MSH colour spaces for such a simple task, but if you want good quality colour maps it's really worth it.
Other than that, I don't know of any 'manual' implementation of the function (i.e. not using already complex functions from matlab or R)
I think it may be more useful to use HSV color space as opposed to RGB, and show your data using the Hue component. This way all the values of your function will map to a nice rainbow color and will be evenly saturated.
In the provided links you should be able to derive the formula, how to convert the Hue value to RGB.
I'm developing a software that detects boxers punching motion. At the moment i used color based segmentation using inRange function and set it to detect blue Minimum value and Blue Maximum value. The problem is that the range is quite wide and my cam at times picks out noise and segments objects of no interest. To improve the software i though of scanning image of a boxing glove and establishing exact Blue color Value before further processing.
It would make sens to me to store that value in a Vector and call it in inRange fiction
// My current function which takes the Minimum and Maximum values of Blue Color
Mat range_out;
inRange(blur_out, Scalar(100, 100, 100), Scalar(120, 255, 255), range_out);
So i would image the vector to go somewhere here.
Scan this above image compute the Blue value
Store this value in an array
recall the array in a inRange function
Could someone suggest a solution to this problem or direct me to a source of information where I can look for answers ?
since you are detecting the boxer gloves in motion so first use motion to separate it from other elements in the scene...use frame differentiation or optical flow to separate the glove and other moving areas from non moving areas...now in those moving area try for some colour detection...
Separe luminosity and cromaticity - your fixed range will not work very well in different light conditions. Your range is wide probably because you are trying to see "blue" in dark and on light at the same time. Convert your image to HSV (or La*b*) and discard V (or L), keeping H and S (or a* and b*).
Learn a color distribution instead a simple range - take some samples and compute a 2D
color histogram on H and S (a* or b*) for pixels on the glove. This histogram will be a model for the color distribution of your object. Then, use c2.calcBackProjection to detect the pixels of interest in your scene.
Clean the result using morphological close operation
Important: on step 2, play a little with different quantization values (ie, different numbers of bins).