CODEWITHSUNDEEP
python-3.xopenglqt5qt3dpyside2
Matphy
Matphy

Reputation: 1134

How to get screen (widget) coordinates from world coordinates

Let's say I have an entity's global (world) coordinate v (QVector3D). Then I make a coordinate transformation:

pos = camera.projectionMatrix() * camera.viewMatrix() * v

where projectionMatrix() and viewMatrix()are QMatrix4x4 instances. What do I actually get and how is this related to widget coordinates?

Upvotes: 1

Views: 863

Answers (1)

Tare
Tare

Reputation: 616

The following values are for OpenGL. They may differ in other Graphics APIs

You get Clip Space coordinates. Imagine a Cube with side length 2 ( i.e. -1 to 1 -w to w on all axes1). You transform your world to have everything you see with your camera in this cube, so that the graphics card can discard everything outside the cube (since you don't see it, it doesn't need rendering. this is for perforamnce reasons).

Going further, you (or rather your graphics API) would do a perspective divide. Then you are in Normalized Device Space - basically here you go from 3D to 2D, such that you know where in your rendering canvas your pixels have to be colored with whatever lighting calculations you use. This canvas is a quad with side length 1 (I believe).

Afterwards you would stretch these normalized device coordinates with whatever width and height your widget has, such that you know where in your widget the colored pixels go (defined in OpenGL as your Viewport).

What you see as Widget Coordinates are probably the coordinates of where your widget is on screen (usually the upper left corner is specified). Therefore, if your widget coordinate is (10, 10) and you have a rendered pixel in your Viewport transformation at (10, 10), then on screen your rendered pixel would be at (10+10, 10+10).

1After having had a discussion with derhass (see comments), a lot of books for graphics programming speak of [-1, -1, -1] x [1, 1, 1] as the clipping volume. The OpenGL 4.6 Core Spec however states that it is actually [-w, -w, -w] x [w, w, w] (and according to derhass, it is the same for other APIs. I have not checked this).

Upvotes: 1

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