I am new to OpenCV so I apologize if I use incorrect terminology. I am writing a program in C that finds objects in an image (in this case red building blocks) and extracts that part of the image and displays it as a new image. I have thresholded the image to remove everything but red and used cvDilate to blur the results slightly to make the object more distinct. I then used the OpenCV Contour finding and drawing functions to locate and draw the blocks.
How can I access the contour locations stored as CvSeq* and take the upper-most and lower-most contour values from a cluster of contours (there may still be some noise from other red objects) so that I can make a bounding box around it?
Thanks
Actually, you don't have to do this manually because OpenCV provides this type of functionality for you.
Look at the cvMinAreaRect2 and cvBoundingRect. Here are their examples respectively: minarea.c (has some debugging stuff, but should give you the gist of how to use it) and generalContours_demo1.cpp (in C++, but should be easy to translate).
As a side note, I would definitely suggest using the C++ API of OpenCV as it is a bit easier to understand and has more features. Also, you spend a lot less time/code worrying about memory management since the Mat class handles that for you.
Hope that helps!
Related
Is there a way to change the clutter perspective for a given container or widget?
The clutter perspective controls how all the clutter actors on the screen are displayed when rotated, translated, scaled, etc.
What I would really like to do is to change the perspective's origin from the center of the screen to another coordinate.
I have messed with a few of the stage methods. However, I haven't had much luck understanding some of the results, and often I hit some stability issues.
I know there are transformation matrices that do all the logic under the hood, and there are documented ways to change the transform matrices. Honestly, I haven't researched much further and just though I would ask for guidance before spending a lot of time on it.
Which leads me to another question regarding the matrices and transformations. Can one of these matrices be used to skew an actor? Or deform it into a trapezoid, etc? And any idea how to get started on that, ie. what a skew matrix would look like?
Finally, does anyone know why the clip path was deprecated? It seems that would have worked for what I ultimately want to do: draw irregular shaped 2d objects on the screen If I can implement an answer to question 2, then I guess a clip box with a transformation can be used here.
1, I do not know if (or how) one might change the Clutter stage's focal point.
2 A skew or shear transformation matrix is easy enough to construct, and can be implemented in the GJS Clutter functions Clutter.Actor.set_transform(T) and Clutter.Actor.set_child_transform(T) where T is a Clutter.Matrix .
This does present another problem, however, for the current codebase; and this leads to another question. (I guess I should post it somewhere else). But, when a transform is set on a clutter actor (or its children), the rest of the actor's properties are ignored. This has the added effect that the Tweener library cannot be used for animation of these properties.
3 Finally, one can use Cairo to draw irregular shaped objects and paths on a Clutter actor, however, the reactive area for the actor (ie. mouse-enter and -leave events) will still be for the entire actor, not defined by the Cairo path.
I am trying to create a bounding box around each character in an image. I have converted the image to binary and thresholded it but I don't understand how to create a bounding box despite reading the manual.
There are a few options for the bounding box technique, but I think you'll get a great result combining these two:
First, use the technique demonstrated here to detect a large portion of text and put a bounding rectangle around it so you crop the image to this area;
Second, experiment with the technique recently presented by OpenCV, also demonstrated here. It could be used to locate/extract individual characters on the resulting image of the first step.
I suppose you are trying to implement the OCR mechanism yourself instead of relying on great APIs such as Tesseract.
If you are looking for more information on how to do digit/text recognition, check this answer.
As said before "rudely", I encourage you to rewrite your question with more detail on what you already did. We didn't understand what you would like to do. If character recognition is what you want, have a look at this.
I have a sequence of images taken from a camera. The images consists of hand and surroundings. I need to remove everything except the hand.
I am new to Image processing. Would anyone help me in regard with the above Question. I am comfortable using C and Matlab.
A really simple approach if you have a stationary background and a moving hand (and quite a few images!) is simply to take the average of the set of images away from each image. If nothing else, it's a gentle introduction to Matlab.
The name of the problem you are trying to solve is "Image Segmentation". The Wikipedia page here: wiki is a good start.
If lighting consistency isn't a problem for you, I'd suggest starting with simple RGB thresholding and see how far that gets you before trying anything more complicated.
Have a look at OpenCV, a FOSS library for computer vision applications. Specifically, see the Video Surveillance module. For a walk through of background subtraction in MATLAB, see this EETimes article.
Can you specify what kind of images you have. Is the background moving or static? For a static background it is a bit straightforward. You simply need to subtract the incoming image from the background image. You can use some morphological operations to make it look better. They all depend on the quality of images that you have. If you have moving background I would suggest you go for color based segmentation. Convert the image to YCbCr then threshold appropriately. I know there are some papers available on it(However I dont have time to locate them). I suggest reading them first. Here is one link which might help you. Read the skin segmentation part.
http://www.stanford.edu/class/ee368/Project_03/Project/reports/ee368group08.pdf
background subtraction is simple to implement (estimate background as average of all frames, then subtract each frame from background and threshold resulting absolute difference) but unfortunately only works well if 1. camera has manual gain and exposure 2. lighting conditions do not change 3.background is stationary. 4. the background is visible for much longer than the foreground.
given your description i assume these are not the case - so what you can use - as already pointed out - is colour as a means of segmenting foreground from background. as it's a hand you are trying to isolate best bet is to learn the hand colour. opencv provides some means of doing this. if you want to do this yourself you just get the colour of some of the hand pixels (you would need to specify this manually for at least one frame) and convert them to HUE (which encapsulates the colour in a brightness independen way. skin colour has a very constant hue) and then make a HUE histogram. compare this to the rest of the pixels and then decided if the hue is simmilar enough.
I am working on a small project where I have to write a low level app. I'd like to display text in that app, and I would even like it to be anti aliased (à la ClearType). No libraries allowed, I have to draw each char pixel by pixel.
What is the best way to do this? Can you recommend some known algorithms? How should I store/read the fonts?
Thanks!
You mean you just want to smooth the edges of an existing bitmapped font? This is easy if your original font is 16x32 and you want to render it at 8x16 or something like that, but if you don't have a higher-resolution bitmap to begin with, smoothing is a highly nontrivial operation involving a lot of guesswork. In that case, I would lookup the 2xsai algorithm (which gives visually-pleasing results for this kind of thing) and first perform it to upscale the font to double resolution, then scale it back down with a area-averaging algorithm (i.e. take each destination pixel from the average of a 4-pixel square).
I would also recommend saving your final "anti-aliased" bitmap font and simply using it in your program, rather than performing all this work at runtime.
Putting all together:
There are two main types of fonts:
1) Monospaced: all the characters have fixed size, and you define a bitmap for each. No need for Anti Aliasing (you can hardcode the grey levels in the bitmap). Look horrible when resized.
2) True Type: each letter is defined by a set of parameters for Bezier curves. Can be easily scaled to any size, but requires lots of program logic (and processing power!) for that. Anti Aliasing is useful here (and especially the sub-pixel rendering techniquies).
As I see you want to use bitmapped font and rescaling? You could just precompute several of them, thus avoiding complex runtime logic.
As R. suggested, keeping the bitmaps at higher resolution in greyscale instead of BW will help. I'd suggest using size that is divisible by most small numbers, so that the bitmap can be downscaled easily. Also, if this resolution is high enough, then you can keep it in BW and downscale to greyscale (using surface integral).
EDIT: feel free to edit it and please don't vote. Just put all those commentaries together.
It is hard to build a good font engine, especially if you need to do scaling and anti-aliasing. So I suggest you take the easy path:
Decide on the fonts and sizes you want to use.
Generate a bitmap font for every font/size combination you need to use. This can be done with a tool like Bitmap Font Generator.
Use the bitmap fonts in your program. Blitting bitmaps should be relatively easy.
If you want more features, I suggest you look into using an engine like FreeType before trying to make your own solution.
Well, reading a TTF (or any other) font and rendering some glyph into the bitmap isnt that hard, given you know some stuff about rasterization and bezier curves. The bad point is that if you want the text to look good, it's gonna take a huge amount of code. Aliased font is pretty hard to render, I'm not talking about hinting. There needs to be a routine for kerning, multi-character sequences, something that decides which glyphs map to your characters and also encoding stuff, ...
You might want to use a bitmap font, which comes pre-rendered - then the whole rendering operation is a simple image copy, eventually with some resampling or rotation; but well, you lose the vector font features.
My advice is to take FreeType and live with it, it's a nice library just for this, and can be statically linked and stripped of unnecessary bloat very easily.
Now that <BevelBitmapEffect> (amongst other effects) has been depreciated, I'm looking to see how I could re-create the exact same thing in a Shader Effect (including it's properties of BevelWidth, EdgeProfile, LightAngle, Relief and Smoothness).
I'm somewhat familar with pixel shading, mostly just colors manipulation of the whole image/element in Shazzam, but how to create a bevel elludes me. Is this a vertex shader and if so, how would I get started? I have searched high and low on this but can't seem to find an inkling of information that would allow me to get started in reproducing <BevelBitmapEffect> in a custom Effect.
Or, based on a comment below, is this 3D in WPF and if so, are there code libraries out there for recreating a <BevelBitmapEffect> that mimics the one that came with previous versions of WPF?
To create the bevel you need to know the distance from the edge for each pixel (search in all directions until alpha=0). From this you can calculate the normal then shade it (see silverlight example). As you mentioned there isn't much content about bevels but there are some good resources if you search for bump mapping/normal mapping to which the shading is similar. In particular this thread has a Silverlight example using a pre-calculated normal map.
To do everything in hardware ideally you would use a multipass shader, WPF's built-in effects are multipass but it doesn't allow you to write your own.
To workaround this limitation:
You could create multiple shaders and nest your element in multiple controls applying a different effect to each one.
Target WPF 4.0 and use Pixel Shader 3.0, for the increased instruction count. Although this may be a too high a hardware requirement and there is no software fallback for PS 3.0
Do some or all of the steps in software.
Without doing one of these you'd be lucky to do a 3 or 4 pixel bevel before you reach the instruction limit as the loops needed to find the distance increase the instruction count quickly.
New Sample
Download. Here is an example that uses PixelShader 3.0. It uses one shader to find the distance (aka height) to the edge, another (based on the nvidia phong shaders) is used to shade it. Bevel profiles are created by adjusting input height either with code or a custom profile can be used by supplying a special texture. There are some other features to add but it seems easily performant enough to animate the properties. Its lacking in comments but I can explain parts if needed.
There's a great article by Rod Stephens on DevX that shows how to use System.Drawing to create the WPF effects (the ones that used to exist, such as Bevel) and more. You've gotta register to view the article though, it's at http://www.devx.com/DevXNet/Article/45039. Downloadable source code too.