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NanoVG

NanoVG is small antialiased vector graphics rendering library for OpenGL. It has lean API modeled after HTML5 canvas API. It is aimed to be a practical and fun toolset for building scalable user interfaces and visualizations.

Screenshot

screenshot of some text rendered witht the sample program

Usage

The NanoVG API is modeled loosely on HTML5 canvas API. If you know canvas, you're up to speed with NanoVG in no time.

Creating drawing context

The drawing context is created using platform specific constructor function. If you're using the OpenGL 2.0 back-end the context is created as follows:

#define NANOVG_GL2_IMPLEMENTATION	// Use GL2 implementation.
#include "nanovg_gl.h"
...
struct NVGcontext* vg = nvgCreateGL2(NVG_ANTIALIAS | NVG_STENCIL_STROKES);

The first parameter defines flags for creating the renderer.

NVG_ANTIALIAS means that the renderer adjusts the geometry to include anti-aliasing. If you're using MSAA, you can omit this flags.
NVG_STENCIL_STROKES means that the render uses better quality rendering for (overlapping) strokes. The quality is mostly visible on wider strokes. If you want speed, you can omit this flag.

Currently there is an OpenGL back-end for NanoVG: nanovg_gl.h for OpenGL 2.0, OpenGL ES 2.0, OpenGL 3.2 core profile and OpenGL ES 3. The implementation can be chosen using a define as in above example. See the header file and examples for further info.

NOTE: The render target you're rendering to must have stencil buffer.

Drawing shapes with NanoVG

Drawing a simple shape using NanoVG consists of four steps: 1) begin a new shape, 2) define the path to draw, 3) set fill or stroke, 4) and finally fill or stroke the path.

nvgBeginPath(vg);
nvgRect(vg, 100,100, 120,30);
nvgFillColor(vg, nvgRGBA(255,192,0,255));
nvgFill(vg);

Calling nvgBeginPath() will clear any existing paths and start drawing from blank slate. There are number of number of functions to define the path to draw, such as rectangle, rounded rectangle and ellipse, or you can use the common moveTo, lineTo, bezierTo and arcTo API to compose the paths step by step.

Understanding Composite Paths

Because of the way the rendering backend is build in NanoVG, drawing a composite path, that is path consisting from multiple paths defining holes and fills, is a bit more involved. NanoVG uses even-odd filling rule and by default the paths are wound in counter clockwise order. Keep that in mind when drawing using the low level draw API. In order to wind one of the predefined shapes as a hole, you should call nvgPathWinding(vg, NVG_HOLE), or nvgPathWinding(vg, NVG_CW) after defining the path.

nvgBeginPath(vg);
nvgRect(vg, 100,100, 120,30);
nvgCircle(vg, 120,120, 5);
nvgPathWinding(vg, NVG_HOLE);	// Mark circle as a hole.
nvgFillColor(vg, nvgRGBA(255,192,0,255));
nvgFill(vg);

Rendering is wrong, what to do?

make sure you have created NanoVG context using one of the nvgCreatexxx() calls
make sure you have initialised OpenGL with stencil buffer
make sure you have cleared stencil buffer
make sure all rendering calls happen between nvgBeginFrame() and nvgEndFrame()
to enable more checks for OpenGL errors, add NVG_DEBUG flag to nvgCreatexxx()
if the problem still persists, please report an issue!

OpenGL state touched by the backend

The OpenGL back-end touches following states:

When textures are uploaded or updated, the following pixel store is set to defaults: GL_UNPACK_ALIGNMENT, GL_UNPACK_ROW_LENGTH, GL_UNPACK_SKIP_PIXELS, GL_UNPACK_SKIP_ROWS. Texture binding is also affected. Texture updates can happen when the user loads images, or when new font glyphs are added. Glyphs are added as needed between calls to nvgBeginFrame() and nvgEndFrame().

The data for the whole frame is buffered and flushed in nvgEndFrame(). The following code illustrates the OpenGL state touched by the rendering code:

	glUseProgram(prog);
	glBlendFunc(GL_SRC_ALPHA, GL_ONE_MINUS_SRC_ALPHA);
	glEnable(GL_CULL_FACE);
	glCullFace(GL_BACK);
	glFrontFace(GL_CCW);
	glEnable(GL_BLEND);
	glDisable(GL_DEPTH_TEST);
	glDisable(GL_SCISSOR_TEST);
	glColorMask(GL_TRUE, GL_TRUE, GL_TRUE, GL_TRUE);
	glStencilMask(0xffffffff);
	glStencilOp(GL_KEEP, GL_KEEP, GL_KEEP);
	glStencilFunc(GL_ALWAYS, 0, 0xffffffff);
	glActiveTexture(GL_TEXTURE0);
	glBindBuffer(GL_UNIFORM_BUFFER, buf);
	glBindVertexArray(arr);
	glBindBuffer(GL_ARRAY_BUFFER, buf);
	glBindTexture(GL_TEXTURE_2D, tex);
	glUniformBlockBinding(... , GLNVG_FRAG_BINDING);

API Reference

See the header file nanovg.h for API reference.

Ports

Projects using NanoVG

License

The library is licensed under zlib license Fonts used in examples:

Roboto licensed under Apache license
Entypo licensed under CC BY-SA 4.0.
Noto Emoji licensed under SIL Open Font License, Version 1.1

Discussions

NanoVG mailing list

Links

Uses stb_truetype (or, optionally, freetype) for font rendering. Uses stb_image for image loading.