sokol_gfx.h: Partial buffer/image update

[lithiumtoast]

Currently, I'm not aware if there is a way to do partial updates with an image or buffer. What I'm looking for is something like the following:

void sg_update_buffer(sg_buffer buf, const void* data_ptr, int data_size, int start_index);

My use case is I have a large 2d texture like a texture atlas or tiled map which I only want to update a sub-region once per frame.

Is there already a way to do what I want with the current API?

[floooh]

I haven't come up with a good way to do partial image updates yet, currently the only solution is to maintain a complete copy of the image data in memory, accumulate partial updates into this with the CPU, and then do a complete sg_update_image().

This is basically what fontstash is doing to dynamically update the font atlas, it maintains a complete CPU-side copy of the font atlas, and sets a dirty flag to tell the backend API to update the font texture:

sokol/util/sokol_fontstash.h

Lines 1000 to 1011 in 57515a1

	SOKOL_API_IMPL void sfons_flush(FONScontext* ctx) {
	SOKOL_ASSERT(ctx && ctx->params.userPtr);
	_sfons_t* sfons = (_sfons_t*) ctx->params.userPtr;
	if (sfons->img_dirty) {
	sfons->img_dirty = false;
	sg_image_content content;
	memset(&content, 0, sizeof(content));
	content.subimage[0][0].ptr = ctx->texData;
	content.subimage[0][0].size = sfons->width * sfons->height;
	sg_update_image(sfons->img, &content);
	}
	}

For buffers, there's an "append API" (sg_append_buffer), this allows to append new data to a buffer and interleave the data updates with draw calls using that newly appended data. This is only useful for per-frame updated data though (it uses the MAP_WRITE_DISCARD strategy in D3D11).

Long term I've got some ideas how to clean up the data update functions (add a map/unmap API for transient per-frame data, and a copy/blit API for "persistent" uploads into buffers and images), but not today or tomorrow.