View Full Version : Drawing rectangular tiles

03-05-2015, 01:46 AM
Hi everybody !

I am writing an application on Android.

In this application, I want to display on the screen a set of rectangular tiles, that act as "big pixels".
I have around 200x150 = 30000 tiles. But it could be higher !

Each tile has two attributes: a color and a visibility flag.
Tiles do not move, but they can, at any moment:
- change color,
- appear or disappear.

Here is how I have implemented these tiles ...
Each tile is composed by two triangles.
Each triangle is a set of three vertex.
Each vertex is composed by:
- a coordinate x, y (2 floats)
- an rgb color (3 floats),
- a visibility flag (1 float)

In the vertex shader, if the visibility flag has a certain value, I move the vertex very far. So it is not shown. I hope the fragment shader is not called.
If it has the correct value, I set the vertex coordinates and color.

In the application, after setting the buffers, I only do:

GLES20.glDrawArrays(GLES20.GL_TRIANGLES, 0, mNbVertices);

And it works well and fast !!!!

I was asking myself if I can improve this design, as I find that there is a lot of redundancy.
The position part of some vertex, could be common to four tiles, which can mean 8 triangles ...

Do you have any tips for improving this design ?



Agent D
03-05-2015, 03:48 AM
First of all, if you want "big pixels", why don't you use "big pixels" by switching to a lower resolution?

Second, your memory usage could be improved by using an indexed rendering (https://www.opengl.org/wiki/Indexed_Rendering), i.e. you have
a vertex buffer with a vertex for each corner of a "big pixel" and an index buffer specifying the vertex indices to use for assembling triangles.

03-05-2015, 04:39 AM
Thanks for your reply !

First, these tiles cover only a part of my screen to simulate an old computer screen. And this screen can move/rotate in 3d space. And I do not how to change the resolution of an android device.

Second, indexed rendering would by okay as it shares vertex, but what about the color ? In fact I would like to share only the positions part of vertex, and not the color or visiblity state (each pixel has its own color).

Agent D
03-05-2015, 04:57 AM
Considering your use case, why don't you simply use a textured rectangle? You draw stuff locally in memory and update portions of
the texture that change. You use nearest-neighbour sampling for the texture to get your desired I-can-see-the-pixels blockyness and
only need to draw two triangles.

03-05-2015, 05:18 AM
That is what I have done first. It works very well on some tablets (Nexus 7), and lags on a "new galaxy tab 10". With profiling I detect the the texture update is the main consumer. Problem of power 2 texture sizes ? Video driver ? That is why I move to rendering triangles.