Develop Reference

How to only focus on the positive Y-axis portion of graph?

I am new to OpenGL, and have recently successfully drawn my first shapes with the guide on the Android Developers website. Right now I am trying to only focus on the upper half of the graph on my OpenGL 2D rendering. So as you guys know, right smack in the middle of the screen is (0,0). However now I want the origin to be at the middle of the bottom of the screen, while maintaining the y axis 0.5f value to be at the middle of the screen and 1f at the top of the screen. In essence, the negative y axis portion is not in view.
These are the coordinates of my square:
float squareCoords[] = {
// x , y , z
-0.5f, 0.5f , 0.0f //top left
-0.5f, 0f , 0.0f //bottom left
0.5f, 0f , 0.0f //bottom right
0.5f, 0.5f , 0.0f //top right
};
This is how I want the square to look on the screen
Ive tried using the camera to focus view but it makes the view bigger(the max y-value and x-value increases, and the object becomes smaller) in the renderer class:
Matrix.setLookAtM(viewMatrix,0,0,0.5f,3,0f,0.5f,0f,0f,1.0f,0.0f)
Does it have something to with GLES20.glViewport? The best I can come up with from online research is a function ortho2d() but it seems like Android Studio does not support it. Any help appreciated.Thanks!

You can use Matrix.orthoM
Matrix.orthoM(
projectionMatrix, 0,
left, right,
bottom, top,
near, far
);
And multiply it with viewMatrix set with Matrix.setLookAtM to obtain View-Projection matrix:
Matrix.multiplyMM(
vpMatrix, 0,
projectionMatrix, 0, viewMatrix, 0
);
Then use vpMatrix in your shader
void main() {
gl_Position = u_VpMatrix * a_Position;
}
...
int uVpMatrix = glGetUniformLocation(program, "u_VpMatrix");
...
glUniformMatrix4fv(uVpMatrix, 1, false, vpMatrix, 0);

Found the soln after further trial and error
In the renderer class at onSurfaceChanged(GL10 unused, int width ,int height){
//edit the Matrix.frustumM method, value for parameter bottom should be set to 0
Matrix.frustumM(projectionMatrix,0,left,right,bottom,top,near,far)
}
Should help with people who want to change the position of their origins to any other location as well, simply change the left,right,top,bottom values.
or refer to this stackoverflow: Set origin to top-left corner of screen in OpenGL ES 2
for other possible solutions.

Finding/Remapping bounds of OpenGL ES coordinate plane

I'm trying to make 2D graphics for my Android app that consists of six thin rectangles that each take up about 1/6th of the screen in width and equal the screen's height. I'm not sure the right way to determine the bounds of the x and y OpenGL coordinate plane on screen. Eventually I will need to write logic that tests which of the 6 rectangles a touch event occurs in, so I have been trying to solve this problem by remapping OpenGL's coordinate plane into the device's screen coordinate plane (where the origin (0,0) is at the top left of the screen instead of the middle.
I declare one of my six rectangles like so:
private float vertices1[] = {
2.0f, 10.0f, 0.0f, // 0, Top Left
2.0f, -1.0f, 0.0f, // 1, Bottom Left
4.0f, -1.0f, 0.0f, // 2, Bottom Right
4.0f, 10.0f, 0.0f, // 3, Top Right
};
but since i'm not sure what the visible limits are on the x and y planes (in the OpenGL coordinate system) I have no concrete way of knowing what vertices my rectangle needs to be instantiated with to occupy 1/6th of the display. Whats the ideal way to do this?
I've tried approaches such as using glOrthoof() to remap OpenGL's coordinates into easy to work with device screen coordinates:
gl.glViewport(0, 0, width, height);
// Select the projection matrix
gl.glMatrixMode(GL10.GL_PROJECTION);
// Reset the projection matrix
gl.glLoadIdentity();
// Calculate the aspect ratio of the window
GLU.gluPerspective(gl, 45.0f,(float) width / (float) height,0.1f, 100.0f);
gl.glOrthof(0.0f,width,height, 0.0f, -1.0f, 5.0f);
// Select the modelview matrix
gl.glMatrixMode(GL10.GL_MODELVIEW);
// Reset the modelview matrix
gl.glLoadIdentity();
but when I do my rectangle dissapears completely.

You certainly don't want to use a perspective projection for 2D graphics. That just doesn't make much sense. A perspective projection is for... well, creating a perspective projection, which is only useful if your objects are actually placed in 3D space.
Even worse, you have two calls to set up a perspective matrix:
GLU.gluPerspective(gl, 45.0f,(float) width / (float) height,0.1f, 100.0f);
gl.glOrthof(0.0f,width,height, 0.0f, -1.0f, 5.0f);
While that's legal, it rarely makes sense. What essentially happens if you do this is that both projections are applied in succession. So the first thing to do is get rid of the gluPerspective() call.
To place your 6 rectangles, you have a few options. Almost the easiest one is to not apply any transformations at all. This means that you will specify your input coordinates in normalized device coordinates (aka NDC), which is a range of [-1.0, 1.0] in both the x- and y-direction. So for 6 rectangles rendered side by side, you would use a y-range of [-1.0, 1.0] for all the rectangles, and an x-range of [-1.0, -2.0/3.0] for the first, [-2.0/3.0, -1.0/3.0] for the second, etc.
Another option is that you use an orthographic projection that makes specifying the rectangles even more convenient. For example, a range of [0.0, 6.0] for x and [0.0, 1.0] for y would make it particularly easy:
gl.glOrthof(0.0f, 6.0f, 0.0f, 1.0f, -1.0f, 1.0f);
Then all rectangles have a y-range of [0.0, 1.0], the first rectangle has a x-range of [0.0, 1.0], the second rectangle [1.0, 2.0], etc.
BTW, if you're just starting with OpenGL, I would pass on ES 1.x, and directly learn ES 2.0. ES 1.x is a legacy API at this point, and I wouldn't use it for any new development.

Android OpenGL ES 2.0 Calculating distance based on screen resolution

I'm trying to draw rectangles with a certain distance in x-axis to each other.
The distance on each rectangle should be
(screen width - rectangle width)
so that only one rectangle can be fully displayed on the screen at a time. And if I drag the first rectangle to left, the portion of that rectangle not displayed (or cut) on the screen would be the amount displayed on second rectangle .
The problem is that I could not figure out how to calculate the ratio of my rectangle's width to screen width.
My rectangle vertices are
final float[] rectangleVerticesData = {
// X, Y, Z
-1.0f, 2.0f, 0.0f,
-1.0f, -2.0f, 0.0f,
1.0f, 2.0f, 0.0f,
-1.0f, -2.0f, 0.0f,
1.0f, -2.0f, 0.0f,
1.0f, 2.0f, 0.0f
};
Based above, my rectangle width is 2. I am using a tablet with 1280 pixels width.
Using the formula above, I got
1280 - 2 = 1278
When I ran the program, the expected output is not as what I am trying to achieve. When I drag the first rectangle, the next rectangle is not displayed even if the first rectangle is fully dragged to left off the screen.
When I tried hard-coding distance to 10, the rectangles are displayed with that distance. I guess 1278 is so long to be the distance between them. How can I calculate the correct distance between the triangles based on screen width so that only one rectangle can be fully displayed at a time on the screen?
Any help would be greatly appreciated. Thank you.

In OpenGL, the screen is considered as a quad of normalised size ie, it extends from vertex (-1,-1) to vertex (+1,+1). The vertex input given to OpenGL for drawing does not range according to the screen pixel size, but is converted internally for drawing to actual screen size. Since there are many devices with varying screen sizes, to make the GL code run across all devices without hardcoding screen sizes, this is required.
So you will have to scale your calculations accordingly.

Android OpenGL Coordinates Question

I am trying to understand the basic concepts around the co-ordinates system in OpenGL so I have been making a test application from guides online.
Presently I have drawn a simple Square to the screen, using simple Co-ordinates of:
-1.0f, 1.0f, 0.0f, // 0, Top Left
-1.0f, -1.0f, 0.0f, // 1, Bottom Left
1.0f, -1.0f, 0.0f, // 2, Bottom Right
1.0f, 1.0f, 0.0f, // 3, Top Right
In my application I run the following code:
GLU.gluPerspective(gl, 45.0f, (float) width / (float) height, 0.1f, 100.0f);
My basic understanding here is that the code is setting the viewing port angle to 45 degrees and the width to height ratio of the window size.
Another thing I am doing is setting the viewing position as -4 units on the Z axis:gl.glTranslatef(0, 0, -4);
This is what the result looks like in Landscape...
And in Portrait...
My questions are:
How does the co-ordinate system work, how many Pixels does one unit represent? How does changing the orientation and width to height ratio effect the equation?
If I wanted to draw a square the size of the screen, with a View Port of 45 degrees and a Viewing position of z-4... how does one figure out the required width and height in units?

I'll try to answer the best I can.
There wouldn't be any reason to change the width to height ratio or 45 degree angle. Doing it the way you have it keeps the things from being stretched horizontally or vertically in an unusual way. Because you are using a perspective view, you have 3D space with depth as apposed to an Orthographic view where there is no depth. In doing glTranslatef(0,0,-4) what you've actually done is changed the MODELVIEW Matix, moving it 4 in the negative z direction, presumably before actually drawing the square. By default, the "camera" is sitting at 0,0,0 with Y (up) as the upward direction.
You may be able to translate 3D space to pixels, but with a Perspective view type, I'm not at all sure you'd really want or need to. A 2D Orthographic view would be a different story, though, as many people use OpenGL for 2D games as well. Wanting a square exactly the size of the screen, Orthographic is probably the way to go, and you should be able to with a few Google searches be able to figure out your pixel density to 2d space comparison.

draw opengl texture at full screen

I want to draw opengl texture at full screen.
(texture : 128x128 ===> device screen : 320x480)
Below code works good, but texture is small.
I have to use only glFrustumf function(not glOrthof function).
How can I draw texture in full screen size?
// this is android source code
float ratio = (float) screenWidth / screenHeight;
gl.glMatrixMode(GL10.GL_PROJECTION);
gl.glLoadIdentity();
gl.glFrustumf(-ratio, ratio, -1, 1, 1, 10);
GLU.gluLookAt(gl, 0.0f, 0.0f, -2.5f, // eye
0.0f, 0.0f, 0.0f, // center
0.0f, 1.0f, 0.0f); // up
// draw blah blah

Why do you have to use glFrustum only? Switching to glOrtho for drawing the background, then switching to glFrustum for regular drawing would be the canonical solution.
BTW: gluLookAt must happen in the modelview matrix, not in the projection matrix like you do right now. As it stands your code is broken and if you were a student in one of my OpenGL classes I'd give you negative points for this cardinal error.