Open GL Line Draw android

Question

I am very new to Open GL. My motto is to create freehand drawing using open GL. This I am trying to achieve through connecting vertices on Drag. My problem is as soon I tap anywhere on screen a line is drawn from center os screen to that point. I am not able to figure out why? Please help.

This is what I am doing to draw :

@Override
public void onDrawFrame(GL10 gl) {
    gl.glClear(GL10.GL_COLOR_BUFFER_BIT);

    glUniform4f(uColorLocation, 1.0f, 0.0f, 0.0f, 1.0f);

    vertexData.put(toFloatarray(points));
    vertexData.clear();

    for (int i = last_size; i < points.size(); i++) {

        glDrawArrays(GL_LINES, i , 2);
        last_size = points.size();

    }

}

I know for loop is not the best way and this is the issue but I am not able to get over it.

EDIT:

This is how I am adding points to array

public void handleTouchDrag(float normalizedX, float normalizedY) {
    points.add(normalizedX);
    points.add(normalizedY);

}

where :

final float normalizedX = (event.getX() / (float) v.getWidth()) * 2 - 1;
final float normalizedY = -((event.getY() / (float) v.getHeight()) * 2 - 1);

https://developer.android.com/training/graphics/opengl/draw.html — Rohan Pawar, Aug 08 '16 at 07:04
see this link http://stackoverflow.com/questions/16650419/draw-in-canvas-by-finger-android — Vishal Thakkar, Aug 08 '16 at 07:21
@AndroidDeveloper thanks but i need to do it in open gl not canvas — Payal, Aug 08 '16 at 08:54
Can you show the code on how you are inserting points to the array. — codetiger, Aug 08 '16 at 13:44

codetiger · Answer 1 · 2016-08-09T04:30:35.597

1

You are feeding the function with only one vertex and requesting to draw a line segment, and OpenGL decides to consider the second point to be origin.

You can use

 glDrawArrays(GL_LINE_STRIP, 0, points.size());

And this will draw lines to previous point.

edited Aug 09 '16 at 04:30

answered Aug 09 '16 at 04:16

codetiger

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Better remove the for loop and try this glDrawArrays(GL_LINE_STRIP, 0, points.size()); I've also edited my code in the answer. – codetiger Aug 09 '16 at 04:31
I have uploaded screenshot of result that I am getting both ways, with or without For loop..I only dragged and drew the bottom line but the center one connected the bottom one is drawing on its own. Please help – Payal Aug 09 '16 at 04:42
If you are drawing GL_LINES, you are suppose to give 2 vertices (x1, y1, x2, y2). and GL_LINE_STRIP will drawling from previous vertex. – codetiger Aug 09 '16 at 04:45

Open GL Line Draw android

1 Answers1