Android: Canvas vs OpenGL

18
votes

I have a drawing app where the user can draw lines with their finger, adjust the color, thickness, etc. As the user is drawing, I am converting the massed X/Y points from MotionEvent into SVG Paths, as well as creating Android Path's and then drawing the Android Path's to the screen via a Canvas, and committing the SVG Path's to the app's database.

I am following the model used in FingerPaint, in that the 'in progress' lines are drawn on the fly by repeated calls to invalidate() (and thus, onDraw()), and once the line is complete and a new line is started, the previous line(s) are drawn in onDraw() from the underlying Canvas Bitmap, with in progress lines again generating repeated re-draws.

This works fine in this application - until you start rotating the underlying Bitmap to compensate for device rotation, supporting the ability to 'zoom in' on the drawing surface and thus having to scale the underlying Bitmap, etc. So for example, with the device rotated and the drawing scaled in, when the user is drawing, we need to scale AND rotate our Bitmap in onDraw(), and this is absolutely crawling.

I've looked at a SurfaceView, but as this still uses the same Canvas mechanism, I'm not sure I'll see noticeable improvement... so my thoughts turn to OpenGL. I have read somewhere that OpenGL can do rotations and scaling essentially 'for free', and even seen rumors (third comment) that Canvas may be disappearing in future versions.

Essentially, I am a little stuck between the Canvas and OpenGL solutions... I have a 2D drawing app that seems to fit the Canvas model perfectly when in one state, as there are not constant re-draws going on like a game (for instance when the user is not drawing I don't need any re-drawing), but when the user IS drawing, I need the maximum performance necessary to do some increasingly complex things with the surface...

Would welcome any thoughts, pointers and suggestions.

3
androidopengl-escanvas

3 Answers

6
votes

OpenGL would be able to handle the rotations and scaling easily.

Honestly, you would probably need to learn a lot of OpenGL to do this, specifically related to the topics of:

  • Geometry
  • Lighting (or just disabling it)
  • Picking (selecting geometry to draw on it)
  • Pixel Maps
  • Texture Mapping
  • Mipmapping

Also, learning OpenGL for this might be overkill, and you would have to be pretty good at it to make it efficient.

Instead, I would recommend using the graphic components of a game library built on top of openGL, such as:

4
votes

Well, this question was asked 6 years ago. Maybe Android 4.0 has not come up? Actually, after Android 4.0 the Canvas at android.view.View is a hardware accelerated canvas, which means it is implementd by OpenGL, so you do not need to use another way for performance.

You can see the https://github.com/ChillingVan/android-openGL-canvas/blob/master/canvasglsample/src/main/java/com/chillingvan/canvasglsample/comparePerformance/ComparePerformanceActivity.java to compare the performance of normal canvas in view with GLSurfaceView.

1
votes

You are right that SurfaceView uses Canvas underneath the hood. The main difference is that SurfaceView uses another thread to do the actual drawing, which generally improves performance. It sounds like it would not help you a great deal, though.

You are correct that OpenGL can do rotations very quickly, so if you need more performance that is the way to go. You should probably use GLSurfaceView. The main drawback with using OpenGL is that it is a real pain to do text. Basically you have to (okay, don't have to, but seems to be the best option) render bitmaps of text.