Drawing to Framebuffer

main.c

#include <string.h>
#include <switch.h>

struct color {
    u8 r, g, b, a;
};

uint8_t BlendColor(u32 src, u32 dst, u8 alpha);
void DrawRectangle(u32 x, u32 y, u32 width, u32 height, struct color clr);

u32 fb_width, fb_height;
u8 * framebuffer;

int main(int argc, char **argv)
{
    //gfxInitResolutionDefault();
    gfxInitDefault();

    socketInitializeDefault();
    nxlinkStdio();

    while(appletMainLoop())
    {
        hidScanInput();
        if (hidKeysDown(CONTROLLER_P1_AUTO) >= 1) {
            break;
        }

        framebuffer = gfxGetFramebuffer(&fb_width, &fb_height);
        memset(framebuffer, 0, gfxGetFramebufferSize());

        DrawRectangle(0, 0, fb_width, fb_height, { 255, 0, 0, 255 });

        gfxFlushBuffers();
        gfxSwapBuffers();
        gfxWaitForVsync();
    }

    socketExit();
    gfxExit();
    return 0;
}

u8 BlendColor(u32 src, u32 dst, u8 alpha)
{
    u8 one_minus_alpha = (u8) 255 - alpha;
    return (dst * alpha + src * one_minus_alpha) / (u8) 255;
}

void DrawRectangle(u32 x, u32 y, u32 width, u32 height, struct color clr) {
    for (u32 xx = x; xx - x <= width; xx++) {
        if (xx >= fb_width)
            break;

        for (u32 yy = y; yy - y <= height; yy++) {
            if (yy >= fb_height)
                break;
            
            u32 off = (yy * fb_width + xx) * 4;
            framebuffer[off] = BlendColor(framebuffer[off], clr.r, clr.a); off++;
            framebuffer[off] = BlendColor(framebuffer[off], clr.g, clr.a); off++;
            framebuffer[off] = BlendColor(framebuffer[off], clr.b, clr.a); off++;
            framebuffer[off] = 0xff;
        }
    }
}