Drawing functions work with matrices/images of arbitrary depth. The boundaries of the shapes can be rendered with antialiasing (implemented only for 8-bit images for now). All the functions include the parameter color that uses a rgb value (that may be constructed with CV_RGB or the Scalar constructor ) for color images and brightness for grayscale images. For color images the order channel is normally Blue, Green, Red , this is what imshow() , imread() and imwrite() expect , so if you form a color using Scalar constructor, it should look like:
If you are using your own image rendering and I/O functions, you can use any channel ordering, the drawing functions process each channel independently and do not depend on the channel order or even on the color space used. The whole image can be converted from BGR to RGB or to a different color space using cvtColor() .
If a drawn figure is partially or completely outside the image, the drawing functions clip it. Also, many drawing functions can handle pixel coordinates specified with sub-pixel accuracy, that is, the coordinates can be passed as fixed-point numbers, encoded as integers. The number of fractional bits is specified by the shift parameter and the real point coordinates are calculated as . This feature is especially effective wehn rendering antialiased shapes.
Also, note that the functions do not support alpha-transparency - when the target image is 4-channnel, then the color[3] is simply copied to the repainted pixels. Thus, if you want to paint semi-transparent shapes, you can paint them in a separate buffer and then blend it with the main image.
Draws a circle
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The function circle draws a simple or filled circle with a given center and radius.
Clips the line against the image rectangle
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The functions clipLine calculate a part of the line segment which is entirely within the specified rectangle. They return false if the line segment is completely outside the rectangle and true otherwise.
Draws a simple or thick elliptic arc or an fills ellipse sector.
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The functions ellipse with less parameters draw an ellipse outline, a filled ellipse, an elliptic arc or a filled ellipse sector. A piecewise-linear curve is used to approximate the elliptic arc boundary. If you need more control of the ellipse rendering, you can retrieve the curve using ellipse2Poly() and then render it with polylines() or fill it with fillPoly() . If you use the first variant of the function and want to draw the whole ellipse, not an arc, pass startAngle=0 and endAngle=360 . The picture below explains the meaning of the parameters.
Parameters of Elliptic Arc
Approximates an elliptic arc with a polyline
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The function ellipse2Poly computes the vertices of a polyline that approximates the specified elliptic arc. It is used by ellipse() .
Fills a convex polygon.
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The function fillConvexPoly draws a filled convex polygon. This function is much faster than the function fillPoly and can fill not only convex polygons but any monotonic polygon without self-intersections, i.e., a polygon whose contour intersects every horizontal line (scan line) twice at the most (though, its top-most and/or the bottom edge could be horizontal).
Fills the area bounded by one or more polygons
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The function fillPoly fills an area bounded by several polygonal contours. The function can fills complex areas, for example, areas with holes, contours with self-intersections (some of thier parts), and so forth.
Calculates the width and height of a text string.
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The function getTextSize calculates and returns size of the box that contain the specified text. That is, the following code will render some text, the tight box surrounding it and the baseline:
// Use "y" to show that the baseLine is about
string text = "Funny text inside the box";
int fontFace = FONT_HERSHEY_SCRIPT_SIMPLEX;
double fontScale = 2;
int thickness = 3;
Mat img(600, 800, CV_8UC3, Scalar::all(0));
int baseline=0;
Size textSize = getTextSize(text, fontFace,
fontScale, thickness, &baseline);
baseline += thickness;
// center the text
Point textOrg((img.cols - textSize.width)/2,
(img.rows + textSize.height)/2);
// draw the box
rectangle(img, textOrg + Point(0, baseline),
textOrg + Point(textSize.width, -textSize.height),
Scalar(0,0,255));
// ... and the baseline first
line(img, textOrg + Point(0, thickness),
textOrg + Point(textSize.width, thickness),
Scalar(0, 0, 255));
// then put the text itself
putText(img, text, textOrg, fontFace, fontScale,
Scalar::all(255), thickness, 8);
Draws a line segment connecting two points
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The function line draws the line segment between pt1 and pt2 points in the image. The line is clipped by the image boundaries. For non-antialiased lines with integer coordinates the 8-connected or 4-connected Bresenham algorithm is used. Thick lines are drawn with rounding endings. Antialiased lines are drawn using Gaussian filtering. To specify the line color, the user may use the macro CV_RGB(r, g, b) .
Class for iterating pixels on a raster line
class LineIterator
{
public:
// creates iterators for the line connecting pt1 and pt2
// the line will be clipped on the image boundaries
// the line is 8-connected or 4-connected
// If leftToRight=true, then the iteration is always done
// from the left-most point to the right most,
// not to depend on the ordering of pt1 and pt2 parameters
LineIterator(const Mat& img, Point pt1, Point pt2,
int connectivity=8, bool leftToRight=false);
// returns pointer to the current line pixel
uchar* operator *();
// move the iterator to the next pixel
LineIterator& operator ++();
LineIterator operator ++(int);
// internal state of the iterator
uchar* ptr;
int err, count;
int minusDelta, plusDelta;
int minusStep, plusStep;
};
The class LineIterator is used to get each pixel of a raster line. It can be treated as versatile implementation of the Bresenham algorithm, where you can stop at each pixel and do some extra processing, for example, grab pixel values along the line, or draw a line with some effect (e.g. with XOR operation).
The number of pixels along the line is store in LineIterator::count .
// grabs pixels along the line (pt1, pt2)
// from 8-bit 3-channel image to the buffer
LineIterator it(img, pt1, pt2, 8);
vector<Vec3b> buf(it.count);
for(int i = 0; i < it.count; i++, ++it)
buf[i] = *(const Vec3b)*it;
Draws a simple, thick, or filled up-right rectangle.
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The function rectangle draws a rectangle outline or a filled rectangle, which two opposite corners are pt1 and pt2 .
Draws several polygonal curves
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The function polylines draws one or more polygonal curves.
Draws a text string
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The function putText renders the specified text string in the image. Symbols that can not be rendered using the specified font are replaced by question marks. See getTextSize() for a text rendering code example.