[ Question ] TTF in OpenGLES2 about simple-gl-evdev

[Miouyouyou]

I'll try to write a quick one.

While I have a library to display TTF fonts, it's horrid, so let me see if I can hack around freetype-gl to display some nice text on the screen, using this example.

The whole ideas, anyway, to display TTF fonts on the screen are "simple" :

1. You either render a complete sentence into a texture and apply this texture to a rectangle on the screen. (SDL-TTF can help in that regard).
2. Or you generate a texture "atlas", which is composed of :

• A texture where the characters are packed near to each other (with at least 1 pixel, or if possible, 2 transparent pixels on each side, to avoid texture filtering issues)
• A data file telling you where the characters are on the texture, and their properties

Then to render a sentence :

• You prepare and send to the GPU a shader that will display "textured rectangles"
• You send the font texture to the GPU
• You prepare a memory buffer with rectangles that will be sent to the GPU
• You start with a virtual pen, at a specific point (x, y).
• For each character :
  • You look up the props of that character in your data file
  • Add a rectangle to your rectangles memory buffer, with the size and the texture coordinates of the character
  • Advance your pen by the "advance x" property of the character
  • When encountering a carriage return, you go back to the beginning (x = line_start_x) and increase (or decrease depending on your matrix) the pen 'y'.
• You send the buffer to the GPU
• Then draw it.

The only issue with this method is how the fonts are stored in the texture. If you generate a simple texture, with bitmap fonts provided by Freetype, and then display the texture parts (characters) using their actual size (meaning display a 7x15 pixels character as 7x15 pixels on the screen), then no 'filtering' will occur and no visual artifact will happen.
If you display them a little bigger or a little smaller than needed, then filtering will happen and things can get ugly.

SDF (Signed Distance Field) fonts textures alleviate this issue by putting a range of pixels around the characters that define how close they are from the actual character borders. Then using a simple shader, and setting up bilinear texture filtering on the texture, for each sampled pixel value :

• a simple color is written on the framebuffer if the pixel value (distance) is equal or higher than a given threshold.
• a 'transparent' pixel is written on the framebuffer if the pixel value is lower than the given threshold
  That gives very crisp and nice results.

Unfortunately, most SDF atlas generators fail to provide a quick and easy way to use them in any C project... Though freetype-gl seems to be the best way for now. Mapbox also have a SDF atlas generator but it's for Javascript (WebGL) projects.

I've generated an atlas with its 'makefont' utility, and I'm trying to get it displayed on the screen... If that fails, I'll try to port my horrid library that still 'kind of work' for simple text purposes.

[avafinger]

I'll try to write a quick one.
That would be awesome. Very nice of you.

I think you mean rendering 2D text something like this:
http://www.opengl-tutorial.org/intermediate-tutorials/tutorial-11-2d-text/

I was thinking of something similar to what the Kodi does, but it looks like a really complex task.

BTW, I tested your example Linux_DRM_OpenGLES.c and it works fine on Kernel 4.4.y and 5.x, so I thought, well, maybe you have something for DRM / GBM or have something in mind.

Thank you.

Cheers.

[Miouyouyou]

Ah, sorry about this one. I admit, I completely forgot about it.

Were you able to find another project ?