Develop Reference

Multiple colors for single path Android

Good day.I am creating a siri like wave for android and i encounter an big issue.I need the wave to be in 4 colors.Lets assume i only have one single line which is drawing on the screen accordingly to the voice decibels.Anyway i am able to do it but no way i am able to give 4 different colors for same path.Assume it is 1 single path which moves from screen start to screen end,i need that line to have 4 different colors,mainly i had to divide the path into 4 parts and draw the color for each parts,but neither google,nor any other source give me anything (not even found anything similar to what i want).
Meanwhile i am posting the code where actually i am drawing the lines.
for (int l = 0; l < mWaveCount; ++l) {
float midH = height / 2.0f;
float midW = width / 2.0f;
float maxAmplitude = midH / 2f - 4.0f;
float progress = 1.0f - l * 1.0f / mWaveCount;
float normalAmplitude = (1.5f * progress - 0.5f) * mAmplitude;
float multiplier = (float) Math.min(1.0, (progress / 3.0f * 2.0f) + (1.0f / 3.0f));
if (l != 0) {
mSecondaryPaint.setAlpha((int) (multiplier * 255));
}
mPath.reset();
for (int x = 0; x < width + mDensity; x += mDensity) {
float scaling = 1f - (float) Math.pow(1 / midW * (x - midW), 2);
float y = scaling * maxAmplitude * normalAmplitude * (float) Math.sin(
180 * x * mFrequency / (width * Math.PI) + mPhase) + midH;
// canvas.drawPoint(x, y, l == 0 ? mPrimaryPaint : mSecondaryPaint);
//
// canvas.drawLine(x, y, x, 2*midH - y, mSecondaryPaint);
if (x == 0) {
mPath.moveTo(x, y);
} else {
mPath.lineTo(x, y);
// final float x2 = (x + mLastX) / 2;
// final float y2 = (y + mLastY) / 2;
// mPath.quadTo(x2, y2, x, y);
}
mLastX = x;
mLastY = y;
}
if (l == 0) {
canvas.drawPath(mPath, mPrimaryPaint);
} else {
canvas.drawPath(mPath, mSecondaryPaint);
}
}
I tried to change color on if (l == 0) {
canvas.drawPath(mPath, mPrimaryPaint);
} but if i change it here,no result at all,either the line is separate and not moving at all,but it should,either the color is not applied,propably because i am doing it in loop as i had to and everytime the last color is picked to draw.Anyway can you help me out?Even an small reference is gold for me because really there is nothing at all in the internet.

Anyway even though Matt Horst answer fully correct,i found the simplest and easiest solution...i never thought it would be so easy.Anyway if in world there is someone who need to make an path divided into multiple colors,here is what you can do
int[] rainbow = getRainbowColors();
Shader shader = new LinearGradient(0, 0, 0, width, rainbow,
null, Shader.TileMode.REPEAT);
Matrix matrix = new Matrix();
matrix.setRotate(90);
shader.setLocalMatrix(matrix);
mPrimaryPaint.setShader(shader);
Where getRainbowColors() is an array of colors you wish your line to have and width is the length of the path so the Shader knows how to draw the colors in right way to fit the length of path.Anyway .....easy isnt it?and pretty purged me a lot to get into this simple point.Nowhere in internet you could find only if you are looking for something completelly different,than you might come across this.

It seems to me like you could set up one paint for each section, each with a different color. Then set up one path for each section too. Then as you draw across the screen, wherever the changeover point is between sections, start drawing with the new path. And make sure first to use moveTo() on the new path so it starts off where the old one left off.

For my solution, I tried changing the color of the linePaint in the onDraw Call. But it was drawing a single color.
So i used two different paints for two different colors and draw path on the canvas.
And it worked. Hope it helps someone out there.

How to determine positions to draw lines on a screen with AndEngine

I have a screen (480x800), M(mx, my) is a static point, and N(nx,ny) is a dynamic point on screen. Position of N(nx, ny) depends position of touch. I want to determine position of P(?,?) and Q(?,?) to draw line 1 and line 2. line 2 is reflective line 1.
This is my code:
private Line l2;
#Override
public boolean onSceneTouchEvent(final Scene pScene,
final TouchEvent pSceneTouchEvent) {
if (this.mPhysicsWorld != null) {
switch (pSceneTouchEvent.getAction()) {
case TouchEvent.ACTION_DOWN:
// Get position
p1x = pSceneTouchEvent.getX();
p1y = pSceneTouchEvent.getY();
return true;
case TouchEvent.ACTION_MOVE:
// Remove instance of the old line
mScene.detachChild(l2);
p3x = pSceneTouchEvent.getX();
p3y = pSceneTouchEvent.getY();
Rectangle testR = new Rectangle(CAMERA_WIDTH / 2,
CAMERA_HEIGHT / 2, 20, 20,
getVertexBufferObjectManager());
l2 = new Line(CAMERA_WIDTH / 2, CAMERA_HEIGHT / 2, p3x, p3y,
getVertexBufferObjectManager());
l2.setColor(new Color(223f / 255f, 118f / 255f, 43f / 255f));
l2.setLineWidth(5);
mScene.attachChild(l2);
return true;
}
return false;
}
return false;
}
If you have other ways to resolve my issue. Please share with me. Thanks.

I didn't program with AndEngine for a while, but this isn't about AndEngine so I can give you a psedu code to solve your problem.
1) Check that nx < mx
2) Calculate the slope of line 1 :
line1slope = (my-ny)/(mx-nx)
3) Find P coordinates using the equation : y-y1 = m(x-x1)
Where m = line1slope
y1 = my
x1 = mx
y = 480 (P's y coordinate) ( if line1slope > 0 then y = 0)
And then you can find your x ( which is P's x coordinate)
line2slope = -1 * line1slope because they are reflective
Now again you need to find Q (You know the X = 0 so you need to find only the Y coordinate)
using the equation : y-y1 = m(x-x1)
Where m = line2slope
y1 = py
x1 = px
x = 0
And then you can find your y coordinate(which is Q's y coordinate)
Hope it helps.

Detect road lanes with android and opencv

i have problem with detection road lanes with my phone.
i wrote some code for road lanes detection, but him not working for me.
From camera get modifications from normal view to BGR colors and try use GausianBlur and Canny, but i think i not good draw lanes for detection.
Maybe some people have another idea how detection road lanes with OpenCV?
Mat mYuv = new Mat(height + height / 2, width, CvType.CV_8UC1);
Mat mRgba = new Mat(height + height / 2, width, CvType.CV_8UC1);
Mat thresholdImage = new Mat(height + height / 2, width, CvType.CV_8UC1);
mYuv.put(0, 0, data);
Imgproc.cvtColor(mYuv, mRgba, Imgproc.COLOR_YUV420p2BGR, 4);
//convert to grayscale
Imgproc.cvtColor(mRgba, thresholdImage, Imgproc.COLOR_mRGBA2RGBA, 4);
// Perform a Gaussian blur (convolving in 5x5 Gaussian) & detect edges
Imgproc.GaussianBlur(mRgba, mRgba, new Size(5,5), 2.2, 2);
Imgproc.Canny(mRgba, thresholdImage, VActivity.CANNY_MIN_TRESHOLD, VActivity.CANNY_MAX_THRESHOLD);
Mat lines = new Mat();
double rho = 1;
double theta = Math.PI/180;
int threshold = 50;
//do Hough transform to find lanes
Imgproc.HoughLinesP(thresholdImage, lines, rho, theta, threshold, VActivity.HOUGH_MIN_LINE_LENGTH, VActivity.HOUGH_MAX_LINE_GAP);
for (int x = 0; x < lines.cols() && x < 1; x++){
double[] vec = lines.get(0, x);
double x1 = vec[0],
y1 = vec[1],
x2 = vec[2],
y2 = vec[3];
Point start = new Point(x1, y1);
Point end = new Point(x2, y2);
Core.line(mRgba, start, end, new Scalar(255, 0, 0), 3);
}

This approach is fine and I've done something similar, not for road line detection but I did notice that it could be used for that purpose. Some comments:
Not sure why you do:
Imgproc.cvtColor(mRgba, thresholdImage, Imgproc.COLOR_mRGBA2RGBA, 4);
as 1. the comment say convert to greyscale, which is a single channel and 2. thresholdImage will get overwritten with the call to Canny later. You just need to dimension thresholdImage with:
thresholdImage = new Mat(mRgba.size(), CvType.CV_8UC1);
What are your parameter values to the call to Canny? I played about with mine considerably and ended up with values like: threshold1 = 441, threshold2 = 160, aperture = 3.
Likewise Imgproc.HoughLinesP: I use Imgproc.HoughLines rather than Imgproc.HoughLinesP with parameters: threshold = 80, minLen = 30, maxLen = 10.
Also have a look at:
for (int x = 0; x < lines.cols() && x < 1; x++){
&& x < 1 means you will only take the first line that the call to HoughLinesP returns. I'd suggest you remove this and use some other criteria to reduce the number of lines; for example, I was interesting in only horizontal and vertical lines so I used atan2 to calculate line angles and exclude those that deviate too much.
UPDATE
Here is how I get the angle of a line. Assuming coordinates of one point is (x1,y1) and the other (x2, y2) then to get the angle:
double lineAngle = Math.atan2(y2 - y1, x2 - x1);
this should return an angle in radians between -PI/2 and PI/2
With regard Canny parameters then I would experiment - I set up onTouch so that I could adjust the threshold values by touching certain parts of the screen to see the effects in realtime. Note that aperture is rather disappointing parameter: it seems to only like odd values 3, 5 and 7 and 3 is the best that I've found.
Something like in the onTouch method:
int w = mRgba.width();
int h = mRgba.height();
float x = event.getX();
float y = event.getY();
if ((x < w / 3) && (y < h / 2)) t1 += 20;
if ((x < w / 3) && (y >= h / 2)) t1 -= 20;
if ((x > 2 * w / 3) && (y < h / 2)) t2 += 20;
if ((x > 2 * w / 3) && (y >= h / 2)) t2 -= 20;
t1 and t2 being the threshold values passed to the Canny call.

Drawing an arrow head in android

I am trying to draw an arrow to point to objects in am image. I have been able to write code to draw the line but cant seem to be able to find a way to draw the arrowhead.The code I wrote to draw a dragabble line is as follows.I need to draw an arrowhead on ACTION_UP event to the direction in which the line is pointing
if(event.getAction() ==MotionEvent.ACTION_DOWN) {
if (count==1){
x1 = event.getX();
y1 = event.getY();
System.out.println(count+"count of value a;skd");
Toast.makeText(getApplicationContext(), ""+(radius+count), Toast.LENGTH_LONG).show();
Log.i(TAG, "coordinate x1 : "+String.valueOf(x1)+" y1 : "+String.valueOf(y1));
}
}
else if(event.getAction() ==MotionEvent.ACTION_MOVE){
imageView.setImageBitmap(bmp2);
x2 = event.getX();
y2 = event.getY();
posX=(float)(x1+x2)/2;
posY=(float)(y1+y2)/2;
radius=(float) Math.sqrt((x1-x2)*(x1-x2) + (y1-y2)*(y1-y2))/2;
onDraw();
Toast.makeText(getApplicationContext(), ""+radius, Toast.LENGTH_LONG).show();
}
Hi, for anyone still needing help .This is how I did it in the end
float h=(float) 30.0;
float phi = (float) Math.atan2(y2 - y1, x2 - x1);
float angle1 = (float) (phi - Math.PI / 6);
float angle2 = (float) (phi + Math.PI / 6);
float x3 = (float) (x2 - h * Math.cos(angle1));
float x4 = (float) (x2 - h * Math.cos(angle2));
float y3 = (float) (y2 - h * Math.sin(angle1));
float y4 = (float) (y2 - h * Math.sin(angle2));
c.drawLine(x1, y1,x2,y2 ,pnt);
c.drawLine(x2, y2,x3,y3 ,pnt);
c.drawLine(x2, y2,x4,y4 ,pnt);
I got help from the accepted answer and ios section in stackoverflow

How I would do this is to find the slope of the line, which is drawn between two points(start and end). The slope would be (dy/dx), and that would be a good start point for your arrow. Assuming you want the base of the arrowhead to be perpendicular to the line of the arrow, to find the slope of the base you would find the opposite reciprocal of the slope of the line. for example, lets say that your line has a slope of 2. The slope for the base of your triangle would be (-1/2), because you do (1/(oldslope)) and multiply by -1. I don't know android very well, but if I remember correctly, in Java, you would use a drawPolygon method, and you would have to specify 4 points(3 unique and 1 the same as the first to close it). Given the slope of the base of the tip, we can get our first two points and our final point. You should know before you start the dimensions of the arrowhead you wish to draw, so in this case b will be the length of your baseline. If you take ϴ=arctan(dy/dx), that will give you an angle between the x axis and your baseline. With that ϴ value, you can do ydif = b*sin(ϴ) to get the difference in y value between the two base corners of your arrow. Doing the same thing but with xdif = b*cos(ϴ) gives you the difference in the x value between the two base points. If the location of the final point of the line that the user drew is, say, (x1, y1), then the locations of the basepoints of the triangle would be (x1-(xdif/2), y1-(ydif/2)) and (x1+(xdif/2), y1+(ydif/2)). These two points, p1 and p2, are the first, second, and fourth points in the draw polygon method. To find the third point, we need to find the angle of the original line, by doing ϴ=arctan(dy/dx), this time using your original dy/dx. with that angle. Before we finish the actual calculation of the point, you first have to know how far from the end of your line the tip of the arrow should actually be, in my case, I will use the var h and h = 10. To get the cordinate, (x,y), assuming the cordinate for the line tip is (x1, y1)you would do (x1+hcosϴ, y1+hsinϴ). Use that for the third value in drawPolygon(), and you should be done. sorry if I kind of rushed at the end, I got kind of tired of typing, comment if you need help.

If you managed to draw a line from the input event, you might additionally draw a triangle on its end indicating the direction.
On another project I drew a square everytime a magnetic point on a grid was touched (as you can see here) Sorry I can not provide you any sample code right now. But if that's a suitable approach for you, I might post it later.

Here is a good code, its not mine, It was a Java Graphics2D code that I converted to Canvas. All credit go to the original guy/lady who wrote it
private void drawArrowHead(Canvas canvas, Point tip, Point tail)
{
double dy = tip.y - tail.y;
double dx = tip.x - tail.x;
double theta = Math.atan2(dy, dx);
int tempX = tip.x ,tempY = tip.y;
//make arrow touch the circle
if(tip.x>tail.x && tip.y==tail.y)
{
tempX = (tip.x-10);
}
else if(tip.x<tail.x && tip.y==tail.y)
{
tempX = (tip.x+10);
}
else if(tip.y>tail.y && tip.x==tail.x)
{
tempY = (tip.y-10);
}
else if(tip.y<tail.y && tip.x==tail.x)
{
tempY = (tip.y+10);
}
else if(tip.x>tail.x || tip.x<tail.x)
{
int rCosTheta = (int) ((10)*Math.cos(theta)) ;
int xx = tip.x - rCosTheta;
int yy = (int) ((xx-tip.x)*(dy/dx) + tip.y);
tempX = xx;
tempY = yy;
}
double x, y, rho = theta + phi;
for(int j = 0; j < 2; j++)
{
x = tempX - arrowLength * Math.cos(rho);
y = tempY - arrowLength * Math.sin(rho);
canvas.drawLine(tempX,tempY,(int)x,(int)y,this.paint);
rho = theta - phi;
}
}
Just call this for both sides of your line and it will draw an arrow at each side!

Why is algorithm so slow?

I'm creating a line graph, and the code I originally used was so slow at drawing that it was useless. I replaced it with code I found online and it became much faster. I was just curious as to why the original code is so slow. All of the code posted below is inside the onDraw() method of a custom view:
Original slow code:
float yStart = 300f;
for (int i=0; i < values.length; i++){
drawPath.moveTo(xStart, yStart);
drawPath.lineTo(xStart+10, values[i]);
drawPath.close();
canvas.drawPath(drawPath, linePaint);
xStart += 10;
yStart = values[i];
}
Later fast code:
float datalength = values.length;
float colwidth = (width - (2 * border)) / datalength;
float halfcol = colwidth / 2;
float lasth = 0;
for (int i = 0; i < values.length; i++) {
float val = values[i] - min;
float rat = val / diff;
float h = graphHeight * rat;
if (i > 0)
canvas.drawLine(((i - 1) * colwidth) + (horStart + 1) + halfcol, (border - lasth) + graphHeight, (i * colwidth) + (horStart + 1) + halfcol, (border - h) + graphHeight, linePaint);
lasth = h;
I just don't understand why one is so much more efficient than the other. Any ideas?

It is CLEAR
In the first piece, there are three operations on objects {moveTo, lineTo, drawPath, and close}
Int the second piece, it is all float operations except one operation on objects

Using Paths makes the drawing significantly slower than simply telling the Canvas to draw a straight line between two points since Path is a much more complex object than the 2 points that drawLine() uses. Paths are also filled and framed based on the Style in the Paint which could also cause a slowdown.
In general, using objects and calling a lot of methods in a loop slows down your code.

After a bit of search, the problem probably came from where I said : you should only call moveTo for the first point of the graph, and then only call lineTo in the loop. When the path is defined entirely (after the for loop) you may draw it. The path is optimized for your purpose, but you where not using it correctly.
http://developer.android.com/reference/android/graphics/Path.html#lineTo%28float,%20float%29

I think you should do it like this:
float yStart = 300f;
drawPath.moveTo(xStart, yStart);
for (int i=0; i < values.length; i++){
drawPath.lineTo(xStart+10, values[i]);
xStart += 10;
yStart = values[i];
}
drawPath.close();
canvas.drawPath(drawPath, linePaint);
otherwise you will draw on the canvas the "building" of drawPath X times.
Also you can precalculate the path and have only the canvas.drawPath in the onDraw.

Also, you should use path.reset() to clear up the memory. Otherwise, every time you draw a different object using the same path it's gonna draw ALL of the objects you've drawn before using that path. So the complete code, would be.
float yStart = 300f;
drawPath.moveTo(xStart, yStart);
for (int i=0; i < values.length; i++){
drawPath.lineTo(xStart+10, values[i]);
xStart += 10;
yStart = values[i];
}
drawPath.close();
canvas.drawPath(drawPath, linePaint);
drawPath.reset();