OpenGL & Qt : display a label on the mouseover

Question

I'm working with OpenGL and Qt. I render a scene in an OpenGLWidget. When hovering over objects in the scene, I would like to display a box near the selected object with some text. I have already implemented the selection of the object.

I thought of two possible approaches.

1. Place a widget (such as a QLabel) above the OpenGLWidget in which the scene is rendered.
2. Render the text in a quad directly in OpenGL.

Which of the two approaches you recommend and you could please give me some suggestions on implementation. Alternatively, you could recommend another approach. Thanks!

Answer 1

Hi @Artic I am not a Qt expert so I can't give you information on widgets, but I can give you some pointers for creating a label with OpenGL. Giving a full implementation is tricky here because it depends a lot on how you want to display the text. But I'll try to outline some of your options.

To render text in OpenGL most people go with a technique known as bitmap fonts, see more here: https://learnopengl.com/In-Practice/Text-Rendering The concept of bitmap fonts is fairly straight forward, all characters are pre-rasterized to a texture and then you can sample from each part of the texture depending on the character you need. You build your label out of quads, textured with each part of the bitmap you sample from for each character.

Signed distance fields essentially use the same technique but the pre-rasterized texture of characters are rendered using signed distance fields which deal with some of the issues that standard bitmaps fonts have.

In basic terms, SDF works by generating a special texture, or image, of the font that stores the distance from the edge of each character to its centre, using the colour channels of the image to record the data.

If you use signed distance fields it won't be enough to just sample from your bitmap, fonts rendered this way require extra work (typically done using a shader program) to produce the correct rendering.

Once you have a way of generating a label you can decide if you want to display it in screen space or in world space.

If you want to display it in world space (where the label is hovering over the model or item) you will need to do more work if you want that label to always face the camera and this technique is called billboarding.

You could also render your text "on the fly" if you just want to render some text to the screen in screen space. You can use a library like SDL_ttf. See: http://lazyfoo.net/tutorials/SDL/16_true_type_fonts/index.php

In this example you use SDL_ttf to render a string of text to a surface with dimensions of your choosing, you can then create an OpenGL texture from that surface and render it to the screen.

Sorry if this information is a bit broad, I would need a more specific question to give you further implementation details.

For an implementation, I would evaluate the pros and cons based on what you need. If you haven't implemented a system for rendering text before it's probably best to stick with something simple; there are more techniques for text rendering than I have listed here such as turning text in to polygons and other libraries which attempt to deal with some of the issues with traditional font rendering techniques but you probably don't need anything complicated.

For a recommendation on which to use I would go with the technique that you feel most comfortable with, typically doing things from scratch in OpenGL will take more time but it can provide you with a nicer set of functionality to use in the future. However if Qt already has something nice for rendering a label (such as a widget that you mentioned) it is probably worth taking the time to learn how to use it as it may yield faster results and you don't want to reinvent the wheel if you don't need to. On that note though doing things from scratch with OpenGL can be very rewarding and greatly improve your understanding since you have to get familiar with how things are done when you don't have a layer of abstraction to depend on. Ultimately it depends on you. Good luck!