Antialiasing in OpenGL ES 2.0?

问题:

Is there a way to implement Anitaliasing technique in OpenGL ES 2.0 ? I have goggled and found few methods but there was no change in the output.

In the worst case I've planned to implement multiple pass rendering, to smooth the edges in fragment shader, by displaying average colour of the pixels around every pixel, but it costs more GPU performance.

Any suggestions ?

回答1:

A lot of devices support MSAA (Multi-Sample Anti-Aliasing). To take advantage of this feature, you have to choose a EGLConfig that has multisampling.

On Android, if you use GLSurfaceView, you will have to implement your own EGLConfigChooser. You can then use EGL functions, particularly eglChooseConfig() to find a config you like.

The following code is untested, but it should at least sketch how this can be implemented. In the constructor of your GLSurfaceView derived class, before calling setRenderer(), add:

setEGLConfigChooser(new MyConfigChooser());

Then implement MyConfigChooser. You can make this a nested class inside your GLSurfaceView:

class MyConfigChooser implements GLSurfaceView.EGLConfigChooser {
    @Override
    public EGLConfig chooseConfig(EGL10 egl, EGLDisplay display) {
        int attribs[] = {
            EGL10.EGL_LEVEL, 0,
            EGL10.EGL_RENDERABLE_TYPE, 4,  // EGL_OPENGL_ES2_BIT
            EGL10.EGL_COLOR_BUFFER_TYPE, EGL10.EGL_RGB_BUFFER,
            EGL10.EGL_RED_SIZE, 8,
            EGL10.EGL_GREEN_SIZE, 8,
            EGL10.EGL_BLUE_SIZE, 8,
            EGL10.EGL_DEPTH_SIZE, 16,
            EGL10.EGL_SAMPLE_BUFFERS, 1,
            EGL10.EGL_SAMPLES, 4,  // This is for 4x MSAA.
            EGL10.EGL_NONE
        };
        EGLConfig[] configs = new EGLConfig[1];
        int[] configCounts = new int[1];
        egl.eglChooseConfig(display, attribs, configs, 1, configCounts);

        if (configCounts[0] == 0) {
            // Failed! Error handling.
            return null;
        } else {
            return configs[0];
        }
    }
}

You will obviously want to substitute the specific values you need for your configuration. In reality, it's much more robust to call eglChooseConfig() with a small set of strictly necessary attributes, let it enumerate all configs that match those attributes, and then implement your own logic to choose the best among them. The defined behavior of eglChooseConfig() is kind of odd already (see documentation), and there's no telling how GPU vendors implement it.

On iOS, you can set this property on your GLKView to enable 4x MSAA:

[view setDrawableMultisample: GLKViewDrawableMultisample4X];

There are other antialiasing approaches you can consider:

• Supersampling: Render to a texture that is a multiple (typically twice) the size of your final render surface in each direction, and then downsample it. This uses a lot of memory, and the overhead is substantial. But if it meets your performance requirements, the quality will be excellent.
• Old school: Render the frame multiple times with slight offsets, and average the frames. This was commonly done with the accumulation buffer in the early days of OpenGL. The accumulation buffer is obsolete, but you can do the same thing with FBOs. See the section "Scene Antialiasing" under "The Framebuffer" in the original Red Book for a description of the method.

回答2:

On Android platform, you can download this OpenGL demo apps source code from GDC 2011: it contains lots of best practices and show you how to do multisampling, including coverage antialiasing.

What you need to do is just customize GLSurfaceView.EGLConfigChooser and set this chooser:

// Set this chooser before calling setRenderer()
setEGLConfigChooser(new MultisampleConfigChooser());
setRenderer(mRenderer);

MultisampleConfigChooser.java sample code below:

package com.example.gdc11;

import javax.microedition.khronos.egl.EGL10;
import javax.microedition.khronos.egl.EGLConfig;
import javax.microedition.khronos.egl.EGLDisplay;

import android.opengl.GLSurfaceView;
import android.util.Log;

// This class shows how to use multisampling. To use this, call
//   myGLSurfaceView.setEGLConfigChooser(new MultisampleConfigChooser());
// before calling setRenderer(). Multisampling will probably slow down
// your app -- measure performance carefully and decide if the vastly
// improved visual quality is worth the cost.
public class MultisampleConfigChooser implements GLSurfaceView.EGLConfigChooser {
    static private final String kTag = "GDC11";
    @Override
    public EGLConfig chooseConfig(EGL10 egl, EGLDisplay display) {
        mValue = new int[1];

        // Try to find a normal multisample configuration first.
        int[] configSpec = {
                EGL10.EGL_RED_SIZE, 5,
                EGL10.EGL_GREEN_SIZE, 6,
                EGL10.EGL_BLUE_SIZE, 5,
                EGL10.EGL_DEPTH_SIZE, 16,
                // Requires that setEGLContextClientVersion(2) is called on the view.
                EGL10.EGL_RENDERABLE_TYPE, 4 /* EGL_OPENGL_ES2_BIT */,
                EGL10.EGL_SAMPLE_BUFFERS, 1 /* true */,
                EGL10.EGL_SAMPLES, 2,
                EGL10.EGL_NONE
        };

        if (!egl.eglChooseConfig(display, configSpec, null, 0,
                mValue)) {
            throw new IllegalArgumentException("eglChooseConfig failed");
        }
        int numConfigs = mValue[0];

        if (numConfigs <= 0) {
            // No normal multisampling config was found. Try to create a
            // converage multisampling configuration, for the nVidia Tegra2.
            // See the EGL_NV_coverage_sample documentation.

            final int EGL_COVERAGE_BUFFERS_NV = 0x30E0;
            final int EGL_COVERAGE_SAMPLES_NV = 0x30E1;

            configSpec = new int[]{
                    EGL10.EGL_RED_SIZE, 5,
                    EGL10.EGL_GREEN_SIZE, 6,
                    EGL10.EGL_BLUE_SIZE, 5,
                    EGL10.EGL_DEPTH_SIZE, 16,
                    EGL10.EGL_RENDERABLE_TYPE, 4 /* EGL_OPENGL_ES2_BIT */,
                    EGL_COVERAGE_BUFFERS_NV, 1 /* true */,
                    EGL_COVERAGE_SAMPLES_NV, 2,  // always 5 in practice on tegra 2
                    EGL10.EGL_NONE
            };

            if (!egl.eglChooseConfig(display, configSpec, null, 0,
                    mValue)) {
                throw new IllegalArgumentException("2nd eglChooseConfig failed");
            }
            numConfigs = mValue[0];

            if (numConfigs <= 0) {
                // Give up, try without multisampling.
                configSpec = new int[]{
                        EGL10.EGL_RED_SIZE, 5,
                        EGL10.EGL_GREEN_SIZE, 6,
                        EGL10.EGL_BLUE_SIZE, 5,
                        EGL10.EGL_DEPTH_SIZE, 16,
                        EGL10.EGL_RENDERABLE_TYPE, 4 /* EGL_OPENGL_ES2_BIT */,
                        EGL10.EGL_NONE
                };

                if (!egl.eglChooseConfig(display, configSpec, null, 0,
                        mValue)) {
                    throw new IllegalArgumentException("3rd eglChooseConfig failed");
                }
                numConfigs = mValue[0];

                if (numConfigs <= 0) {
                  throw new IllegalArgumentException("No configs match configSpec");
                }
            } else {
                mUsesCoverageAa = true;
            }
        }

        // Get all matching configurations.
        EGLConfig[] configs = new EGLConfig[numConfigs];
        if (!egl.eglChooseConfig(display, configSpec, configs, numConfigs,
                mValue)) {
            throw new IllegalArgumentException("data eglChooseConfig failed");
        }

        // CAUTION! eglChooseConfigs returns configs with higher bit depth
        // first: Even though we asked for rgb565 configurations, rgb888
        // configurations are considered to be "better" and returned first.
        // You need to explicitly filter the data returned by eglChooseConfig!
        int index = -1;
        for (int i = 0; i < configs.length; ++i) {
            if (findConfigAttrib(egl, display, configs[i], EGL10.EGL_RED_SIZE, 0) == 5) {
                index = i;
                break;
            }
        }
        if (index == -1) {
            Log.w(kTag, "Did not find sane config, using first");
        }
        EGLConfig config = configs.length > 0 ? configs[index] : null;
        if (config == null) {
            throw new IllegalArgumentException("No config chosen");
        }
        return config;
    }

    private int findConfigAttrib(EGL10 egl, EGLDisplay display,
            EGLConfig config, int attribute, int defaultValue) {
        if (egl.eglGetConfigAttrib(display, config, attribute, mValue)) {
            return mValue[0];
        }
        return defaultValue;
    }

    public boolean usesCoverageAa() {
        return mUsesCoverageAa;
    }

    private int[] mValue;
    private boolean mUsesCoverageAa;
}

Before using this feature, you should known this will affect rendering efficiency, and may need to do a fully performance test.