OpenGL绘制一张图片的流程--以android-openGL-canvas为例

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样例代码来源: android-openGL-canvas

OpenGL文档参考：
OpenGL文档
本文只对流程做排序以及一些简单的说明，想了解原理的请查看OpenGL相关文档，有对相应函数有疑问的可查阅上述文档。

一、创建 eglContext。EGLContext 是一个比较重的对象，所以一般只创建一次。

例子代码代码主要在 EglHelper

egl = EGLContext.getEGL()
获取 eglDisplay = eglGetDisplay(EGL10.EGL_DEFAULT_DISPLAY)
获取 EGLConfig eglChooseConfig(..)
eglInitialize(mEgl, mEglDisplay, mEglConfig, eglContext)
创建 eglContext = eglCreateContext(display, config, eglContext, attrib_list))
大致的相关代码

    public EGLContext start(EGLContext eglContext) {        if (GLThread.LOG_EGL) {            Log.w("EglHelper", "start() tid=" + Thread.currentThread().getId());        }        /*         * Get an EGL instance         */        mEgl = (EGL10) EGLContext.getEGL();        /*         * Get to the default display.         */        mEglDisplay = mEgl.eglGetDisplay(EGL10.EGL_DEFAULT_DISPLAY);        if (mEglDisplay == EGL10.EGL_NO_DISPLAY) {            throw new RuntimeException("eglGetDisplay failed");        }        /*         * We can now initialize EGL for that display         */        int[] version = new int[2];        if (!mEgl.eglInitialize(mEglDisplay, version)) {            throw new RuntimeException("eglInitialize failed");        }        mEglConfig = eglConfigChooser.chooseConfig(mEgl, mEglDisplay);            /*            * Create an EGL context. We want to do this as rarely as we can, because an            * EGL context is a somewhat heavy object.            */        mEglContext = eglContextFactory.createContext(mEgl, mEglDisplay, mEglConfig, eglContext);        if (mEglContext == null || mEglContext == EGL10.EGL_NO_CONTEXT) {            mEglContext = null;            throwEglException("createContext");        }        if (GLThread.LOG_EGL) {            Log.w("EglHelper", "createContext " + mEglContext + " tid=" + Thread.currentThread().getId());        }        mEglSurface = null;        return mEglContext;    }

二、创建 surface

eglCreateWindowSurface(display, config, nativeWindow, surfaceAttribs)
将 context 和 surface 联系起来。
eglMakeCurrent(eglDisplay, eglSurface, eglSurface, eglContext)
如果需要更换surface，就先destroy现在的
eglDestroySurface
大致的相关代码

    public boolean createSurface(Object surface) {        if (GLThread.LOG_EGL) {            Log.w("EglHelper", "createSurface()  tid=" + Thread.currentThread().getId());        }        /*         * Check preconditions.         */        if (mEgl == null) {            throw new RuntimeException("egl not initialized");        }        if (mEglDisplay == null) {            throw new RuntimeException("eglDisplay not initialized");        }        if (mEglConfig == null) {            throw new RuntimeException("mEglConfig not initialized");        }        /*         *  The window size has changed, so we need to create a new         *  surface.         */        destroySurfaceImp();        /*         * Create an EGL surface we can render into.         */        mEglSurface = eglWindowSurfaceFactory.createWindowSurface(mEgl,                mEglDisplay, mEglConfig, surface);        if (mEglSurface == null || mEglSurface == EGL10.EGL_NO_SURFACE) {            int error = mEgl.eglGetError();            if (error == EGL10.EGL_BAD_NATIVE_WINDOW) {                Log.e("EglHelper", "createWindowSurface returned EGL_BAD_NATIVE_WINDOW.");            }            return false;        }        /*         * Before we can issue GL commands, we need to make sure         * the context is current and bound to a surface.         */        if (!mEgl.eglMakeCurrent(mEglDisplay, mEglSurface, mEglSurface, mEglContext)) {            /*             * Could not make the context current, probably because the underlying             * SurfaceView surface has been destroyed.             */            logEglErrorAsWarning("EGLHelper", "eglMakeCurrent", mEgl.eglGetError());            return false;        }        return true;    }

三、接下来就是使用vertex数据，texture数据，vertex shader, texture shader进行绘制了，这一部分是可以随时替换数据绘制新的东西的，其中比较耗性能的部分是编译链接shader的部分，所以要对编译链接部分进行缓存以保证性能。

代码主要在GLES20Canvas

vertex 数据

glGenBuffers(…)
glBindBuffer(GLES20.GL_ARRAY_BUFFER, bufferId)
glBufferData(GLES20.GL_ARRAY_BUFFER, buffer.capacity() * elementSize, buffer, GLES20.GL_STATIC_DRAW)

大致的相关代码

    private int uploadBuffer(Buffer buffer, int elementSize) {        mGLId.glGenBuffers(1, mTempIntArray, 0);        checkError();        int bufferId = mTempIntArray[0];        GLES20.glBindBuffer(GLES20.GL_ARRAY_BUFFER, bufferId);        checkError();        GLES20.glBufferData(GLES20.GL_ARRAY_BUFFER, buffer.capacity() * elementSize, buffer,                GLES20.GL_STATIC_DRAW);        checkError();        return bufferId;    }

编译，链接shader等

以两个 String 对象作为参数，分别是 vertex shader 和 fragment shader
编译
glCreateShader
glShaderSource
glCompileShader

    private static int loadShader(int type, String shaderCode) {        // create a vertex shader type (GLES20.GL_VERTEX_SHADER)        // or a fragment shader type (GLES20.GL_FRAGMENT_SHADER)        int shader = GLES20.glCreateShader(type);        // add the source code to the shader and compile it        GLES20.glShaderSource(shader, shaderCode);        checkError();        GLES20.glCompileShader(shader);        checkError();        return shader;    }

链接
glCreateProgram
glAttachShader vertex
glAttachShader fragment
glLinkProgram
glGetProgramiv

    private int assembleProgram(int vertexShader, int fragmentShader, ShaderParameter[] params) {        int program = GLES20.glCreateProgram();        checkError();        if (program == 0) {            throw new RuntimeException("Cannot create GL program: " + GLES20.glGetError());        }        GLES20.glAttachShader(program, vertexShader);        checkError();        GLES20.glAttachShader(program, fragmentShader);        checkError();        GLES20.glLinkProgram(program);        checkError();        int[] mLinkStatus = mTempIntArray;        GLES20.glGetProgramiv(program, GLES20.GL_LINK_STATUS, mLinkStatus, 0);        if (mLinkStatus[0] != GLES20.GL_TRUE) {            Log.e(TAG, "Could not link program: ");            Log.e(TAG, GLES20.glGetProgramInfoLog(program));            GLES20.glDeleteProgram(program);            program = 0;        }        loadHandles(params, program);        return program;    }

设定绘制区域

glViewport(width, height)
此时就可以利用 width 和 height 来对 model view projection 矩阵进行一些初始设置了

    @Override    public void setSize(int width, int height) {        mWidth = width;        mHeight = height;        GLES20.glViewport(0, 0, mWidth, mHeight);        checkError();        Matrix.setIdentityM(mMatrices, mCurrentMatrixIndex);        Matrix.orthoM(mProjectionMatrix, 0, 0, width, 0, height, -1, 1);        if (getTargetTexture() == null) {            mScreenWidth = width;            mScreenHeight = height;            Matrix.translateM(mMatrices, mCurrentMatrixIndex, 0, height, 0);            Matrix.scaleM(mMatrices, mCurrentMatrixIndex, 1, -1, 1);        }    }

对model-view矩阵进行处理(也可以不处理，看需求)

可以是平移，旋转，放缩之类的。

传入 texture, 一个带有宽高和图像数据的对象。

glUseProgram(programId) 使用之前编译链接好的的id
将texture数据传入到OpenGL里的gl操作
glActiveTexture(GLES20.GL_TEXTURE0)

    private void prepareTexture(BasicTexture texture, int program, ShaderParameter[] params) {        GLES20.glUseProgram(program);        checkError();        enableBlending(!texture.isOpaque() || getAlpha() < OPAQUE_ALPHA);        GLES20.glActiveTexture(GLES20.GL_TEXTURE0);        checkError();        texture.onBind(this);        GLES20.glBindTexture(texture.getTarget(), texture.getId());        checkError();        GLES20.glUniform1i(params[INDEX_TEXTURE_SAMPLER].handle, 0);        checkError();        GLES20.glUniform1f(params[INDEX_ALPHA].handle, getAlpha());        checkError();    }

glGenTextures
glBindTexture
glTexParameteri 设定相关的texture params

    @Override    public void setTextureParameters(BasicTexture texture) {        int target = texture.getTarget();        GLES20.glBindTexture(target, texture.getId());        checkError();        GLES20.glTexParameteri(target, GLES20.GL_TEXTURE_WRAP_S, GLES20.GL_CLAMP_TO_EDGE);        GLES20.glTexParameteri(target, GLES20.GL_TEXTURE_WRAP_T, GLES20.GL_CLAMP_TO_EDGE);        GLES20.glTexParameterf(target, GLES20.GL_TEXTURE_MIN_FILTER, GLES20.GL_LINEAR);        GLES20.glTexParameterf(target, GLES20.GL_TEXTURE_MAG_FILTER, GLES20.GL_LINEAR);    }

如果bitmap的宽高是2的倍数
glBindTexture
glTexImage2D

    @Override    public void initializeTexture(BasicTexture texture, Bitmap bitmap) {        int target = texture.getTarget();        GLES20.glBindTexture(target, texture.getId());        checkError();        GLUtils.texImage2D(target, 0, bitmap, 0);    }

如果不是2的倍数
glBindTexture
GLUtils.texSubImage2D

    @Override    public void texSubImage2D(BasicTexture texture, int xOffset, int yOffset, Bitmap bitmap,            int format, int type) {        int target = texture.getTarget();        GLES20.glBindTexture(target, texture.getId());        checkError();        GLUtils.texSubImage2D(target, 0, xOffset, yOffset, bitmap, format, type);    }

glUniform1i(...) // 对shader里面的textureSampler传值，因为ActiveTexture的是Texture0，所以传0就可以了。

    GLES20.glUniform1i(params[INDEX_TEXTURE_SAMPLER].handle, 0);

将vertex的值传入OpenGL

glBindBuffer
glVertexAttribPointer
glBindBuffer 0 清理

    private void setPosition(ShaderParameter[] params, int offset) {        GLES20.glBindBuffer(GLES20.GL_ARRAY_BUFFER, mBoxCoordinates);        checkError();        GLES20.glVertexAttribPointer(params[INDEX_POSITION].handle, COORDS_PER_VERTEX,                GLES20.GL_FLOAT, false, VERTEX_STRIDE, offset * VERTEX_STRIDE);        checkError();        GLES20.glBindBuffer(GLES20.GL_ARRAY_BUFFER, 0);        checkError();    }

texture Matrix

利用texture的宽高设定 textureMatrix。 textureMatrix 是shader 里的一个uniform对象，用来映射从texture对应位置获取的颜色到绘制区域。例如在不进行平移旋转等情况下，第一个像素点就绘制在第一个位置。

        TextureMatrixTransformer.copyTextureCoordinates(...);        TextureMatrixTransformer.convertCoordinate(...);        TextureMatrixTransformer.setTextureMatrix(...);

model view projection matrix 将这个matrix传入到OpenGL里

translateM
scaleM
multiplyMM
glUniformMatrix4fv(…)

    private void setMatrix(ShaderParameter[] params, float x, float y, float width, float height) {        Matrix.translateM(mTempMatrix, 0, mMatrices, mCurrentMatrixIndex, x, y, 0f);        Matrix.scaleM(mTempMatrix, 0, width, height, 1f);        Matrix.multiplyMM(mTempMatrix, MATRIX_SIZE, mProjectionMatrix, 0, mTempMatrix, 0);        GLES20.glUniformMatrix4fv(params[INDEX_MATRIX].handle, 1, false, mTempMatrix, MATRIX_SIZE);        checkError();    }

最后就是绘制了

glEnableVertexAttribArray positionHandle
glDrawArrays
glDisableVertexAttribArray positionHandle

    private void draw(ShaderParameter[] params, int type, int count, float x, float y, float width,            float height) {        setMatrix(params, x, y, width, height);        int positionHandle = params[INDEX_POSITION].handle;        GLES20.glEnableVertexAttribArray(positionHandle);        checkError();        GLES20.glDrawArrays(type, 0, count);        checkError();        GLES20.glDisableVertexAttribArray(positionHandle);        checkError();    }

四、总结

以上，说明了OpenGL绘制的三个流程。由于OpenGL是独立于语言的，这些流程不仅适用于Android，使用其它语言编写也可借鉴。从上面冗长的流程可以看到，每次要进行绘制都写那么多代码实在不经济，那么接下来的文章就会讨论如何封装以上的流程了。其实开头的github仓库里面的代码已经有封装好的类了，有需要的话可以先去看源代码。感谢阅读！

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