实例渲染OpenGL

Question

我正在努力弄清楚为什么对 glDrawArraysInstanced 的调用正在绘制正确数量的实例但是对象的大小错误。 这是我的代码（ommnited class declaration）

#include <QtOpenGL/QGLWidget>
#include <QtOpenGL/QGLBuffer>
#include <QtOpenGL/QGLShaderProgram>
#include <iostream>

const float OpenGLViewer::_quad[] = {
    -1.0f, 0.0f, 0.0f,
    -0.9f, 0.0f, 0.0f,
    -1.0f,  1.0f, 0.0f,
    -0.9f,  1.0f, 0.0f
};

const float OpenGLViewer::_offset[] = {
    0.0, 0.2, 0.7, 0.4
};

...

void OpenGLViewer::initializeGL()
{
    QGLFormat glFormat = QGLWidget::format();
    if(!glFormat.sampleBuffers())
        std::cout << "Could not enable sample buffers.";

    glClearColor(0.0f, 0.0f, 0.4f, 0.0f);
    glViewport(0, 0, this->width(), this->height());

    if(!prepareShaderProgram(":/vertex.glsl", ":/fragment.glsl"))
        return;

    if(!_shaderProgram.bind())
    {
        std::cout << "Could not bind shader program to the context.";
        return;
    }

    glGenVertexArrays(1, &_vertexArraysObject);
    glBindVertexArray(_vertexArraysObject);         // BIND

    glGenBuffers(1, &_quadBuffer);
    glBindBuffer(GL_ARRAY_BUFFER, _quadBuffer);     // BIND
    glBufferData(GL_ARRAY_BUFFER, sizeof(_quad), _quad, GL_STATIC_DRAW);
    glEnableVertexAttribArray(0);
    glVertexAttribPointer(0, 3, GL_FLOAT, GL_FALSE, 0, 0);
    //glVertexAttribDivisorARB(0, 0);

    glGenBuffers(1, &_offsetBuffer);
    glBindBuffer(GL_ARRAY_BUFFER, _offsetBuffer);
    glBufferData(GL_ARRAY_BUFFER, sizeof(_offset), _offset, GL_STATIC_DRAW);
    glEnableVertexAttribArray(1);
    glVertexAttribPointer(1, 1, GL_FLOAT, GL_FALSE, 0, 0);
    glVertexAttribDivisorARB(1, 1);

    glBindVertexArray(0);
    glBindBuffer(GL_ARRAY_BUFFER, 0);
}

void OpenGLViewer::paintGL()
{
    // Clear the buffer with the current clearing color
    glClear( GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT );

    // Draw stuff
    glBindVertexArray(_vertexArraysObject);

    glDrawArraysInstanced(GL_TRIANGLE_STRIP, 0, 4, 4);
}

...

我要做的是使用顶点着色器中的偏移值绘制相同的矩形但位于不同的位置。重要的是要说我使用 glVertexAttribDivisorARB（1,1）（注意ARB后缀）因为在Qt + OSX Mavericks中我找不到函数 glVertexAttribDivisor 。有什么想法吗？

生成的图像看起来像

修改

忘记包含着色器

// VERTEX shader
#version 330

layout(location = 0) in vec4 vertexPosition;
layout(location = 1) in float offset;

void main()
{
    gl_Position = vec4(vertexPosition.x + offset, vertexPosition.y + offset, vertexPosition.zw);
}

//FRAGMENT shader
#version 330

layout(location = 0, index = 0) out vec4 fragmentColor;

void main(void)
{
    fragmentColor = vec4(1.0, 0.0, 0.0, 1.0);
}

Answer 1

gl_Position = vec4(vertexPosition.x + offset, vertexPosition.y + offset, vertexPosition.zw);

将偏移添加到y将其向上移动（屏幕外）

你应该不将偏移量添加到y或者减去它：

gl_Position = vec4(vertexPosition.x + offset, vertexPosition.y - offset, vertexPosition.zw);