画线按在OpenGL屏幕COORDS [英] Drawing lines as per screen coords in OpenGL

问题描述

我有我在OpenGL-ES 2.0已经创建了一个2D地图。现在我要画一个比例尺。

I have a 2D map that I have created in OpenGL-ES 2.0. Now I want to draw a scalebar.

当用户平移地图，这将不会被感动，我需要它在屏幕上绘制坐标弄好了。

As this will not be moved when the user pans the map, I need it to be drawn in screen coordinates somehow.

我可以使用上如何完成这样的一些建议。

I could use some advice on how to accomplish something like this.

推荐答案

要做到这一点，你需要知道的是你的视口的尺寸。要知道，视口变换是一个非常简单的规模和偏见的操作，它有效地是这样的：

To do this, all you need to know is the dimensions of your viewport. Know that the viewport transformation is a very simple scale and bias operation, it effectively works like this:

 NDC    Screen
 ---    ------
-1.0 -> x          \_ X  (Passed using glViewport (...))
 1.0 -> x + width  /

-1.0 -> y          \_ Y  (Passed using glViewport (...))
 1.0 -> y + height /

-1.0 -> near       \_ Z  (Passed using glDepthRange (...))
 1.0 -> far        /

^{我的丑ASCII艺术道歉，我希望堆栈溢出​​支持LaTeX的。}

在换句话说，X和Y，标准化设备坐标（NDC）的缩放到合适的宽度和高度众多单位中，并在的底层最左侧的点是由（x，y）的定义。对于Z，这你可能不关心，NDC是从附近的价值定义的值为止您定义的缩放。

In other words, for X and Y, Normalized Device Coordinates (NDC) are scaled to fit within width and height many units, and the bottom-left-most point is defined by (x,y). For Z, which you probably do not care about, NDC is scaled from your defined near value to the defined far value.

你想要做什么的逆转视映射操作;开始屏幕空间坐标和工作回到NDC。

What you want to do is reverse the viewport mapping operation; start with screenspace coordinates and work back into NDC.

• NDC _X =（2.0 *（屏幕 _X - 视 _X ）/视_是W ） - 1.0;

• NDC_X = (2.0 * (Screen_X - Viewport_X) / Viewport_W) - 1.0;

NDC <子>是 =（2.0 *（屏幕_是 - 视_是）/视 _{^ h} ） - 1.0;

NDC_Y = (2.0 * (Screen_Y - Viewport_Y) / Viewport_H) - 1.0;

这应该为您提供所有必要的步骤来产生NDC <子>（X，Y）从屏幕<子>（X，Y）坐标。一旦你有了NDC坐标，你可以直接通过他们使用的模型，视图和投影矩阵身份着色器。

This should provide you with all the necessary steps to produce NDC_(X,Y) coordinates from Screen_(X,Y). Once you have NDC coordinates, you can pass them directly to your shaders using identity matrices for Model, View and Projection.

<子>的选择是跳过了这一切，并使用一个定义1正交投影矩阵：投影和窗口（另一个名称屏幕空间）坐标1之间的映射，但是我觉得这是更好地教NDC以及如何视口映射工作，从长远来看：）

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