Nexus 7 OpenGL line rasterisation issue

/*
 * Copyright (C) 2009 The Android Open Source Project
 *
 * Licensed under the Apache License, Version 2.0 (the "License");
 * you may not use this file except in compliance with the License.
 * You may obtain a copy of the License at
 *
 *      http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */

// OpenGL ES 2.0 code

#include <jni.h>
#include <android/log.h>

#include <GLES2/gl2.h>
#include <GLES2/gl2ext.h>

#include <stdio.h>
#include <stdlib.h>
#include <math.h>

#define  LOG_TAG    "libgl2jni"
#define  LOGI(...)  __android_log_print(ANDROID_LOG_INFO,LOG_TAG,__VA_ARGS__)
#define  LOGE(...)  __android_log_print(ANDROID_LOG_ERROR,LOG_TAG,__VA_ARGS__)

static void printGLString(const char *name, GLenum s) {
    const char *v = (const char *) glGetString(s);
    LOGI("GL %s = %s\n", name, v);
}

static void checkGlError(const char* op) {
    for (GLint error = glGetError(); error; error
            = glGetError()) {
        LOGI("after %s() glError (0x%x)\n", op, error);
    }
}

static const char gVertexShader[] =
    "attribute vec4 vPosition;\n"
    "void main() {\n"
    "  gl_Position = vPosition;\n"
    "}\n";

static const char gFragmentShader[] =
    "precision mediump float;\n"
    "void main() {\n"
    "  gl_FragColor = vec4(0.0, 1.0, 0.0, 1.0);\n"
    "}\n";

GLuint loadShader(GLenum shaderType, const char* pSource) {
    GLuint shader = glCreateShader(shaderType);
    if (shader) {
        glShaderSource(shader, 1, &pSource, NULL);
        glCompileShader(shader);
        GLint compiled = 0;
        glGetShaderiv(shader, GL_COMPILE_STATUS, &compiled);
        if (!compiled) {
            GLint infoLen = 0;
            glGetShaderiv(shader, GL_INFO_LOG_LENGTH, &infoLen);
            if (infoLen) {
                char* buf = (char*) malloc(infoLen);
                if (buf) {
                    glGetShaderInfoLog(shader, infoLen, NULL, buf);
                    LOGE("Could not compile shader %d:\n%s\n",
                            shaderType, buf);
                    free(buf);
                }
                glDeleteShader(shader);
                shader = 0;
            }
        }
    }
    return shader;
}

GLuint createProgram(const char* pVertexSource, const char* pFragmentSource) {
    GLuint vertexShader = loadShader(GL_VERTEX_SHADER, pVertexSource);
    if (!vertexShader) {
        return 0;
    }

    GLuint pixelShader = loadShader(GL_FRAGMENT_SHADER, pFragmentSource);
    if (!pixelShader) {
        return 0;
    }

    GLuint program = glCreateProgram();
    if (program) {
        glAttachShader(program, vertexShader);
        checkGlError("glAttachShader");
        glAttachShader(program, pixelShader);
        checkGlError("glAttachShader");
        glLinkProgram(program);
        GLint linkStatus = GL_FALSE;
        glGetProgramiv(program, GL_LINK_STATUS, &linkStatus);
        if (linkStatus != GL_TRUE) {
            GLint bufLength = 0;
            glGetProgramiv(program, GL_INFO_LOG_LENGTH, &bufLength);
            if (bufLength) {
                char* buf = (char*) malloc(bufLength);
                if (buf) {
                    glGetProgramInfoLog(program, bufLength, NULL, buf);
                    LOGE("Could not link program:\n%s\n", buf);
                    free(buf);
                }
            }
            glDeleteProgram(program);
            program = 0;
        }
    }
    return program;
}

GLuint gProgram;
GLuint gvPositionHandle;
GLfloat vertices[8];
bool setupGraphics(int w, int h) {
    printGLString("Version", GL_VERSION);
    printGLString("Vendor", GL_VENDOR);
    printGLString("Renderer", GL_RENDERER);
    printGLString("Extensions", GL_EXTENSIONS);

    LOGI("setupGraphics(%d, %d)", w, h);
    gProgram = createProgram(gVertexShader, gFragmentShader);
    if (!gProgram) {
        LOGE("Could not create program.");
        return false;
    }
    gvPositionHandle = glGetAttribLocation(gProgram, "vPosition");
    checkGlError("glGetAttribLocation");
    LOGI("glGetAttribLocation(\"vPosition\") = %d\n",
            gvPositionHandle);

    glViewport(0, 0, w, h);
    checkGlError("glViewport");
    float proj_x = 2.0f/w;
    float proj_y = 2.0f/h;
    float pa = 0.5f * proj_x;
    float pb = 5.5f * proj_x;
    float y = 5.5f  * proj_y;
    float y2 = 6.5f * proj_y;

    //pa to pb
    vertices[0] = pa;
    vertices[1] = y;
    vertices[2] = pb;
    vertices[3] = y;

    //pb to pa
    vertices[4] = pb;
    vertices[5] = y2;
    vertices[6] = pa;
    vertices[7] = y2;

    return true;
}


void renderFrame() {
    glClearColor(0, 0, 0, 1.0f);
    checkGlError("glClearColor");
    glClear( GL_DEPTH_BUFFER_BIT | GL_COLOR_BUFFER_BIT);
    checkGlError("glClear");

    glUseProgram(gProgram);
    checkGlError("glUseProgram");

    glVertexAttribPointer(gvPositionHandle, 2, GL_FLOAT, GL_FALSE, 0, vertices);
    checkGlError("glVertexAttribPointer");
    glEnableVertexAttribArray(gvPositionHandle);
    checkGlError("glEnableVertexAttribArray");
    glDrawArrays(GL_LINES, 0, 8);
    checkGlError("glDrawArrays");
}

extern "C" {
    JNIEXPORT void JNICALL Java_com_android_gl2jni_GL2JNILib_init(JNIEnv * env, jobject obj,  jint width, jint height);
    JNIEXPORT void JNICALL Java_com_android_gl2jni_GL2JNILib_step(JNIEnv * env, jobject obj);
};

JNIEXPORT void JNICALL Java_com_android_gl2jni_GL2JNILib_init(JNIEnv * env, jobject obj,  jint width, jint height)
{
    setupGraphics(width, height);
}

JNIEXPORT void JNICALL Java_com_android_gl2jni_GL2JNILib_step(JNIEnv * env, jobject obj)
{
    renderFrame();
}