Tutorial: How to modify an image to insert basic drawings

Table of Contents

• Introduction
• Modify an image with basic drawings
  • Draw a point in an image
  • Draw a line between 2 points
  • Draw a circle
  • Draw a rectangle
  • Draw a cross
  • Insert text in an image
• Next tutorial

Introduction

In this tutorial you will learn how to modify the content of an image adding basic drawings without the need of an image display window. This functionality could be useful if none of the following 3rd parties are available: X11, GDI, OpenCV, GTK, Direct3D.

Modify an image with basic drawings

There is the vpImageDraw class that allows to modify an image by inserting basic drawings like point, circle, line, rectangle, polygon, frame. There is also vpFont class that allows to modify an image to insert text. These classes are used in testImageDraw.cpp.

If you run the corresponding binary:

$ cd $VISP_WS/visp-build/modules/core
$ ./testImageDraw

it will create canvas_color.png and canvas_gray.png images that give a good overview.

• Content of canvas_color.png image that shows basic drawings inserted in a color image implemented as a vpImage<vpRGBa> is the following:
  
• Content of canvas_gray.png image that shows basic drawings inserted in a gray level image implemented as a vpImage<unsigned char> is the following:
  

Draw a point in an image

The following snippet shows how to modify color image I drawing a red point at pixel location (100, 200).

vpImage<vpRGBa> I(480, 640);
vpImagePoint ip(100, 200);
vpImageDraw::drawPoint(I, ip, vpColor::red);

The following snippet shows how to modify a gray level image I drawing a white point at pixel location (100, 200).

vpImage<unsigned char> I(480, 640);
vpImagePoint ip(100, 200);
unsigned char color = 255; // white
vpImageDraw::drawPoint(I, ip, color);

Draw a line between 2 points

The following snippet shows how to modify color image I drawing an orange line with thickness 3 between pixels with coordinates (100, 200) and (300, 400).

vpImage<vpRGBa> I(480, 640);
vpImagePoint ip1(100, 200);
vpImagePoint ip2(300, 400);
vpImageDraw::drawLine(I, ip1, ip2, vpColor::orange, 3);

The following snippet shows how to modify gray level image I drawing a black line with thickness 3 between pixels with coordinates (100, 200) and (300, 400).

vpImage<unsigned char> I(480, 640);
vpImagePoint ip1(100, 200);
vpImagePoint ip2(300, 400);
unsigned char color = 0; // black
vpImageDraw::drawLine(I, ip1, ip2, color, 3);

Draw a circle

The following snippet shows how to modify color image I drawing a green cercle with thickness 3, centered at pixel location (100, 200) and with radius 80 pixels.

vpImage<vpRGBa> I(480, 640);
vpImagePoint ip(100, 200);
vpImageDraw::drawCircle(I, ip, 80, vpColor::green, 3);

The following snippet shows how to modify gray level image I drawing a gray cercle with thickness 3, centered at pixel location (100, 200) and with radius 80 pixels.

vpImage<unsigned char> I(480, 640);
vpImagePoint ip(100, 200);
unsigned char color = 128; // gray
vpImageDraw::drawCircle(I, ip, 80, color, 3);

Draw a rectangle

The following snippet shows how to modify color image I drawing a yellow rectangle with thickness 3, with top left corner location (100, 200), and rectangle width and height set to 150, 80 respectively.

vpImage<vpRGBa> I(480, 640);
vpImagePoint ip(100, 200);
int w = 150;
int h = 80;
vpImageDraw::drawRectangle(I, vpRect(ip, w, h), vpColor::yellow, false, 3);

The following snippet shows how to modify gray level image I drawing a light gray rectangle with thickness 3, with top left corner location (100, 200), and rectangle width and height set to 150, 80 respectively.

vpImage<unsigned char> I(480, 640);
vpImagePoint ip(100, 200);
int w = 150;
int h = 80;
unsigned char color = 200; // light gray
vpImageDraw::drawRectangle(I, vpRect(ip, w, h), color, false, 3);

Draw a cross

The following snippet shows how to modify color image I drawing a blue cross with thickness 3, location (100, 200), and size 15 pixels.

vpImage<vpRGBa> I(480, 640);
vpImagePoint ip(100, 200);
int w = 150;
int h = 80;
vpImageDraw::drawCross(I, ip, 15, vpColor::blue, 1);

The following snippet shows how to modify gray level image I drawing a dark gray cross with thickness 3, location (100, 200), and size 15 pixels.

vpImage<unsigned char> I(480, 640);
vpImagePoint ip(100, 200);
int w = 150;
int h = 80;
unsigned char color = 50; // dark gray
vpImageDraw::drawCross(I, ip, 15, color, 1);

Insert text in an image

The following snippet shows how to modify color image I drawing "Hello world" in white over a black background at location (100, 200).

vpImage<vpRGBa> I(480, 640);
vpImagePoint ip(100, 200);
font.drawText(I, "Test...", ip, vpColor::white, vpColor::black);

The following snippet shows how to modify gray level image I drawing "Hello world" in white over a black background at location (100, 200).

vpImage<unsigned char> I(480, 640);
vpImagePoint ip(100, 200);
unsigned char color = 255; // white
unsigned char background = 0; // black
font.drawText(I, "Test...", ip, color, background);

Next tutorial

You are now ready to see how to continue with Tutorial: Image frame grabbing.