What are the differences between Soft/Hard mask and fonts generated by bmfont?

[jiangbo]

I see that the coordinates in the font files generated by soft mask are all decimals, with 0.5 decimals, while the text coordinates of bmfont are all integers.
Does the generation by soft mask require a separate msdf shader to render? It can't be rendered using a normal sprite renderer, is there a difference here?

[Chlumsky]

It is this way to make sure that neighboring glyphs don't bleed into the displayed one in case bilinear filtering is used (without the need to make the border 2px). If you use nearest-neighbor filtering and need integer coordinates, then you can enlarge both atlasBounds and planeBounds by 0.5 pixels in each direction (for planeBounds in ems, that would be by 0.5/size ems, or just multiply the bounds by size to get values in pixels). I would also recommend using the option -pxalign on to make sure planeBounds are aligned with the pixel grid too.

[jiangbo]

Sorry, I don't know much about fonts.

1. I understand that when using linear filtering, need to set a border around each character to avoid bleed. What do you mean by setting it to 0.5, which doesn't require a 2px boundary box?
2. Do you suggest using nearest-neighbor filtering to render these fractional coordinates? Is the effect the same as using integer coordinates?
3. I previously used msdf, which requires a separate pixel shader. If I use Soft/Hard, can I directly use a quad sprite renderer?

[Chlumsky]

Integer texture coordinates correspond to halway-points between pixels, while values ending in .5 are the actual centers of pixels. For example, if you have a tiny 2x2 texture, centers of pixels are at [0.5, 0.5], [0.5, 1.5], etc., While the coordinate [1, 1] is exactly in-between the four pixels. Therefore, using .5 texture coordinates makes sure that pure pixel colors are at the perimeter of the quad, which is more important when the texture is being upscaled (typical for SDF). If you wanted to avoid bilinear filter bleed from neighboring glyphs while keeping integer coordinates, the atlas would have to have 2 pixels thick border between glyphs, so that the integer coordinate can be set to the midpoint between the 2 pixels of the border. With a 1 pixel border, the midpoint coordinate ends in .5.

If you're drawing from the atlas at 1:1 scale, you can use nearest-neighbor filtering with both integer and fractional coordinates, you just need to make sure you don't offset the pixel grid (texture vs sceen) by 1/2 of a pixel, which results in bad artifacts. Yes, for sotf/hard mask you can literally just copy the pixels from the atlas to the screen.

[jiangbo]

I kind of understand, .5 is used to get the center of the pixel.

Using .5 texture coordinates makes sure that pure pixel colors are at the perimeter of the quad, --- is it referring to the center here?

But if the pixel is offset by 0.5, what about the last pixel? For example, a 2x2 image, the original sampling points X (0,1,2), after offset (0.5,1.5,2.5), the last point is outside the image.

You just need to make sure you don't offset the pixel grid (texture vs screen) by 1/2 of a pixel, --- This means: If my texture is offset by 0.5, then my screen pixels also need to be offset by 0.5, or both should use integers without offsetting, right?

[Chlumsky]

I'm sorry but if you don't understand that there is no third pixel (in the same direction) in a 2x2 texture, I won't be able to explain this to you. I suggest you just try it out and see if it works for you or not.

[jiangbo]

Thank you for your professional answer.