playing around with 3DS, next step is to add 3D LAYERS. build at https://d.maxfile.ro/uthptfzswu.3dsx

simple-paint.c

#include <string.h>
#include <stdio.h>
#include <stdbool.h>

#include <3ds.h>

// given buffer, its width and height, real coords, and rgb values
// draw corresponding pixel as given rgb color 
// doesn't draw pixel if (x, y) is outside box bounded by (0, 0) (w-1, h-1) rect
bool setPixel(u8 *fb, u16 w, u16 h, u16 r_x, u16 r_y, u8 r, u8 g, u8 b) {
	u16 x = h - r_y - 1;
	u16 y = r_x;
	if (x < h && y < w) {
		fb[3*(x+y*h)] = b;
		fb[3*(x+y*h)+1] = g;
		fb[3*(x+y*h)+2] = r;
		return true;
	} else {
		return false;
	}
}

void drawPlusAtCoord(u8 *fb, u16 w, u16 h, u16 x, u16 y, u8 r, u8 g, u8 b) {
	/* setPixel(fb, w, h, x, y, r, g, b); */
	setPixel(fb, w, h, x+1, y, r, g, b);
	setPixel(fb, w, h, x-1, y, r, g, b);
	setPixel(fb, w, h, x+2, y, r, g, b);
	setPixel(fb, w, h, x-2, y, r, g, b);
	setPixel(fb, w, h, x, y+1, r, g, b);
	setPixel(fb, w, h, x, y-1, r, g, b);
	setPixel(fb, w, h, x, y+2, r, g, b);
	setPixel(fb, w, h, x, y-2, r, g, b);
}

int main()
{
	gfxInitDefault();
	//gfxSet3D(true); // uncomment if using stereoscopic 3D
	PrintConsole console;
	consoleInit(GFX_BOTTOM, &console);
	consoleSelect(&console);

	//cursor psn
	u16 x = 10, y = 10;
	u16 w = 400, h = 240;

	u8 *drawing = linearAlloc(w*h*3);
	memset(drawing, 0x0, w*h*3);

#define COLORCNT 7
	//available colors
	struct {
		u8 r;
		u8 g;
		u8 b;
	} color[COLORCNT];
	color[0].r = 0xFF; color[0].g = 0xFF; color[0].b = 0xFF; //white
	color[1].r = 0xFF; color[1].g = 0x00; color[1].b = 0x00; //red
	color[2].r = 0x00; color[2].g = 0xFF; color[2].b = 0x00; //green
	color[3].r = 0x00; color[3].g = 0x00; color[3].b = 0xFF; //blue
	color[4].r = 0xFF; color[4].g = 0xFF; color[4].b = 0x00; //yellow
	color[5].r = 0x00; color[5].g = 0xFF; color[5].b = 0xFF; //cyan
	color[6].r = 0xFF; color[6].g = 0x00; color[6].b = 0xFF; //magenta

	//selected color
	u8 c = 0;

	// Main loop
	while (aptMainLoop())
	{
		gspWaitForVBlank();
		hidScanInput();

		// Your code goes here

		u32 kDown = hidKeysDown();
		u32 kHeld = hidKeysHeld();

		if (kDown & KEY_DUP || kHeld & (KEY_CSTICK_UP | KEY_CPAD_UP))  {
			y -= 1;
		}
		if (kDown & KEY_DDOWN || kHeld & (KEY_CSTICK_DOWN | KEY_CPAD_DOWN)) {
			y += 1;
		}
		if (kDown & KEY_DLEFT || kHeld & (KEY_CSTICK_LEFT | KEY_CPAD_LEFT)) {
			x -= 1;
		}
		if (kDown & KEY_DRIGHT || kHeld & (KEY_CSTICK_RIGHT | KEY_CPAD_RIGHT)) {
			x += 1;
		}
		if (kHeld & KEY_A) {
			setPixel(drawing, w, h, x, y, color[c].r, color[c].g, color[c].b);
		}
		if (kHeld & KEY_B) {
			setPixel(drawing, w, h, x, y, 0x00, 0x00, 0x00);
		}
		if (kDown & KEY_X) {
			c += 1;
			c %= COLORCNT;
		}
		if (kDown & KEY_Y) {
			c += COLORCNT - 1;
			c %= COLORCNT;
		}
		if (kDown & KEY_SELECT) {
			memset(drawing, 0x0, h*w*3);
		}
		if (kDown & KEY_START) {
			break; // break in order to return to hbmenu
		}

		// ensure x and y are within bounds
		y += h;
		y %= h;
		x += w;
		x %= w;

		// Example rendering code that displays a white pixel
		// Please note that the 3DS screens are sideways (thus 240x400 and 240x320)
		u8 *fb = gfxGetFramebuffer(GFX_TOP, GFX_LEFT, &h, &w);
		consoleClear();
		printf("(%d, %d)\n\n[A] to paint\n[B] to erase\n[Y]/[X] to cycle colors\n[select] to clear\n[start] to exit", x, y);
		memset(fb, 0x0, h*w*3);
		memcpy(fb, drawing, w*h*3);

		drawPlusAtCoord(fb, w, h, x, y, color[c].r, color[c].g, color[c].b);

		// Flush and swap framebuffers
		gfxFlushBuffers();
		gfxSwapBuffers();
	}
	linearFree(drawing);

	gfxExit();
	return 0;
}