Serialize/deserialize font cache to/from memory.

[AlexanderKotliar]

Hello, Jonathan! Thank You for a useful library!

On mobiles it takes a few seconds to generate each font cache. So, it would be nice to have a way to save and load it in memory to be flexible. One can add these memory blocks to the whole application's cache and store it to disk.

Best regards,
Alexander

[grimfang4]

You're welcome! So your suggestion is a sort of format specification so that we can save and load the internal font cache? I like that idea and it would certainly come in handy.

SDL_FontCache uses a set of textures (the font cache levels) to store the glyph graphics and it stores a map of position rects for each loaded glyph. We would need some text format saver/loader and an image saver/loader or else delegate to the application by providing access to these data objects. I definitely prefer the second approach to keep the dependencies clean.

[AlexanderKotliar]

Text saver/loader is too big for such little library.

Can I iterate and get/set internal data by FC_GetGlyphCacheLevel/FC_SetGlyphCacheLevel and FC_GetGlyphData/FC_SetGlyphData?
What is cache level?
What range has codepoint?

[grimfang4]

FC_GetNumCacheLevels() will tell you how many textures the font is holding. Then use FC_GetGlyphCacheLevel() to get each of them.

FC_SetGlyphCacheLevel() can be used when loading to tell the font to use a particular texture you've loaded.

There is not currently a way to get all of the glyph data, so we should add that. Right now, you'd have to know which codepoints have been loaded already in order to use FC_GetGlyphData(). The range for codepoints can be any 4-byte UTF-8 sequence (endianness might matter). Once you have all of that data, you would use FC_MakeGlyphData() and FC_SetGlyphData() for each codepoint.

[grimfang4]

Alright, I just pushed 51f588e. You should be able to use FC_GetNumCodepoints() to size an array, then FC_GetCodepoints() to fill the array with all of the loaded codepoints. I also exposed FC_UploadGlyphCache() so you can pass in SDL_Surfaces instead of textures and the library will do the conversion. This will make it possible to save/load from memory (or other, via SDL_rwops).

[AlexanderKotliar]

Thank You for quick reply, Jonathan!

There's no way to get SDL_Surface from SDL_Texture.
I catch surfaces from the call back just before SDL_FreeSurface. It works, but new symbols isn't rendered, may be due to some internal FC_Font's states was not saved.
I found compromise solution for now - empty loading string.

SDL_GetHint return 0 when no hint is set, so we need to check:

old_filter_mode[256]; // Save it so we can change the hint value in the meantime
char* mode = SDL_GetHint(SDL_HINT_RENDER_SCALE_QUALITY);
(old_filter_mode, 255, "%s", mode ? mode : "");
...
SDL_SetHint(SDL_HINT_RENDER_SCALE_QUALITY, strlen(old_filter_mode) ? old_filter_mode : 0);

I increase buffer size because there are may be such a hint like "linear" etc.

[grimfang4]

Oh yeah, you can use SDL_LockTexture() to get pixels into a surface, but it's not guaranteed to be the latest texture data. You could use a temporary render target and do SDL_RenderReadPixels() from it, which should assure the latest data. I think that's how most people would save an SDL_Texture to file?

[grimfang4]

That sounded like a challenge, so I wrote up this function. It mostly does what I expected, though reuploading displays some sort of precision drift. Something is off a little bit.

SDL_Surface* copy_texture_to_surface(SDL_Renderer* renderer, SDL_Texture* texture)
{
    if(renderer == NULL || texture == NULL)
        return NULL;

    // Ensure texture is ready for copying
    SDL_SetTextureColorMod(texture, 255, 255, 255);
    SDL_SetTextureAlphaMod(texture, 255);

    // Figure out texture pixel format
    Uint32 format;
    int w, h;
    if(SDL_QueryTexture(texture, &format, NULL, &w, &h) < 0)
        return NULL;

    // Prepare surface pixel format
    int bpp;
    Uint32 Rmask, Gmask, Bmask, Amask;
    if(!SDL_PixelFormatEnumToMasks(format, &bpp, &Rmask, &Gmask, &Bmask, &Amask))
        return NULL;

    // Create temp render target
    SDL_Texture* temp = SDL_CreateTexture(renderer, format, SDL_TEXTUREACCESS_TARGET, w, h);
    if(temp == NULL)
        return NULL;

    // Copy the texture
    SDL_SetTextureBlendMode(temp, SDL_BLENDMODE_NONE);
    SDL_SetRenderTarget(renderer, temp);
    SDL_RenderCopy(renderer, texture, NULL, NULL);

    SDL_Surface* surface = SDL_CreateRGBSurface(SDL_SWSURFACE, w, h, bpp, Rmask, Gmask, Bmask, Amask);
    if(surface == NULL)
    {
        SDL_DestroyTexture(temp);
        return NULL;
    }

    // Read the temp pixels
    if(SDL_RenderReadPixels(renderer, NULL, format, surface->pixels, surface->pitch) < 0)
    {
        SDL_FreeSurface(surface);
        SDL_SetRenderTarget(renderer, NULL);
        SDL_DestroyTexture(temp);
        return NULL;
    }

    SDL_SetRenderTarget(renderer, NULL);
    SDL_DestroyTexture(temp);
    return surface;
}

[AlexanderKotliar]

It looks impressive!
It needs to clear render target:
SDL_SetRenderTarget(renderer, temp);
SDL_SetRenderDrawColor(renderer, 0, 0, 0, 255);
SDL_RenderClear(renderer);
SDL_RenderCopy(renderer, texture, NULL, &rect);

And resulting surface is flipped by y. How I can flip it back?

[grimfang4]

You could use SDL_RenderCopyEx() with a vertical flip of the texture render step. Otherwise, you can do it by hand by swapping scanlines in the SDL_Surface. I have to do this in SDL_gpu:
https://github.com/grimfang4/sdl-gpu/blob/master/src/renderer_GL_common.inl#L2428

[AlexanderKotliar]

Thank You, Jonathan!

Now, cache saving is working!
Although I added FC_Get/SetLastGlyph to save current position in texture.

But one problem with copy_texture_to_surface: it slightly blur the image. After a few times small fonts degenerate.
SDL_FontCache creates textures with SDL_SetHint(SDL_HINT_RENDER_SCALE_QUALITY, "0");
At the begin of copy_texture_to_surface:I set it too, but it doesn't help.
How can I fix it?

[grimfang4]

I'm not really sure why this is. It could be a subpixel offset issue? I don't have the time right now to tinker with it.