Normalized Device Coordinates mapping to 2d array indices

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I created a checkerboard/chessboard in opengl, it has 100 boxes. Logically I'm representing that board in a 2D array (10 by 10). Now I want the user to click anywhere in the opengl window and i can determine which index the box belongs to in the array, so that i can do some processing. How can i achieve that?

I'm using LWJGL a java wrapper for opengl. I know i have to get the mouse coordinates and i know they are returned in Normalized Device Coordinates ( in the range of 1 to -1) How can I map NDC to indexes? I've been searching but didn't find much.

I know the NDC ranges for each boxes. That's because i drew them by giving vertices so i know where each box starts and ends in NDC. But entering the ranges and its respective indexes in the form of key-value pairs would be too troublesome.

Currently I'm trying to create a formula through basic arithmetic such that it transforms NDC and gives me indexes. Is this possible? or is there any other feasible approach?

For those who are thinking how i have made the checkerboard, I drew vertical lines from -1 to 1 at a regular distance of 0.2. Similar is the case for horizontal ones.

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javaarraysopengllwjgl

1 Answers

0
votes

If you are drawing directly in screen space it is quite easy to compute the box indices from NDC coords:

index.x = (int) ((ndc.x/2.0f + 0.5f) * boardSize.x)
index.y = (int) ((ndc.y/2.0f + 0.5f) * boardSize.y)
if (index.x == boardSize.x) // in case ndc.x was exactly 1
    --index.x;
if (index.y == boardSize.y) // in case ndc.y was exactly 1
    --index.y;