Hah. I've actually discovered that my layout engine was better written than I thought. It does send entire runs to the OpenType layout engine. But then it digs through the shaped item list and replaces space characters with fixed-width glue nodes. If I just tell it to accept the shaper's understanding of the width of the space rather than use a fixed width, it all works.
I'm not really sure what I'm looking at here. Is it meant to look a bit hand-drawn, a bit crude and bumpy? If so, that's fine, it's a legitimate aesthetic - although perhaps you could rough the /d up a bit; the inside bowl is too lovely. If not, take a look at the curves on, say, the arches of the /n and the /h.
I can't really speak to the Arabic but it seems (to me at least) cleaner and less crude than the Latin. Compare the smoothness of the /dad-ar with the bumpiness of the /R. I don't know if that's a good match. Maybe tell us a bit more about your design idea?
Are the feet of the /d and /a (and the /h and the /n) supposed to look different? Again if you're going for crude and hand-drawn, that's probably OK.
The outer leg join of the /R looks to be at the wrong angle - should be going in to match the lower part of the bowl, so heading south-west instead of west.
Do you have a full Latin alphabet yet or are you still developing it?
On one hand I hear people talking about thousands of kern pairs. On the other I hear people like Andreas saying "space everything as well as you can, then kern only what is necessary", and Bruno Maag saying "kern straight-straight, straight-round, round-straight, round-round and be done with it". I wonder if he's oversimplifying for effect, but I take from that a pushback against over-complicated kerning and over-reliance. I wonder how to resolve this.
I have to be a bit careful saying this, but... I wonder if there is a danger in coming to think of the number of kern pairs as a proxy for font quality. And I wonder if it becomes a pride issue - a way of showing how seriously you are taking things.
I just opened up a bunch of fonts. Three were by truly world class designers, whose names you will know. The first was a display face with no kerning at all, which surprised me. It's still a great font. The second was a text face with 750 pairs (no categories) and all of the kerns were at 10 unit intervals. Applying categories and compressing, I got it down to 345. Another, which is widely regarded as an absolute classic display font, has 890 pairs.
The fourth was by a designer I had never heard of. It has 230 glyphs, and even using categories has 2557 kern pairs, and they're all over the place: six units here, eight units there; /K/V gets -32 units whereas /K/W gets -31. Does that really make a difference? I don't know. But I think the designer was able to satisfy himself that he worked very hard in a way that he would not be satisfied with if he had only 300 pairs. Is there some internal pressure on people to achieve a certain number of kern pairs?
"Space like hell, kern when you must" makes a lot of sense to me. How many "kern pairs" did some of the most famous letterpress types have? And every day I see software that still doesn't apply kerning at all. But still, it's hard for me to avoid the feeling that if I end up with less than five or six hundred pairs I haven't done a good enough job.