Question

我正在研究基于2D纹理的体积渲染项目，当我尝试将alpha和混合应用到我的程序时会出现问题。程序从一个文件中读取2D帧，并根据这些帧设置纹理。这是我的计划：

//
//  VolumeRendering.cpp
//  Volume_Rendering
//
//  Created by HOBBY on 4/5/14.
//  Copyright (c) 2014 Yihao Jiang. All rights reserved.
//
#include <GLTools.h>
#include <GL/glew.h>
#include <Opengl/gl.h>
#include <glut/glut.h>
#include <fstream>
#include "VolumeRendering.h"
#include <GLMatrixStack.h>
#include <GLFrustum.h>
#include <GLGeometryTransform.h>

int m_uImageCount;
int m_uImageWidth;
int m_uImageHeight;
GLuint* m_puTextureIDs;

GLMatrixStack modelViewMatrix;
GLMatrixStack projectionMatrix;
GLFrame       cameraFrame;
GLFrame       objectFrame;
GLFrustum     viewFrustum;
GLBatch       myBatch;
GLGeometryTransform    transformPipeline;
GLShaderManager        shaderManager;

void ChangeSize(int w, int h)
{
    glViewport(0, 0, w, h);
    //viewFrustum.SetPerspective(35.0f, float(w) / float(h), 1.0f, 500.0f);
    viewFrustum.SetOrthographic(-1.0f, 1.0f, -1.0f, 1.0f, -2.0f, 2.0f);
    projectionMatrix.LoadMatrix(viewFrustum.GetProjectionMatrix());
    transformPipeline.SetMatrixStacks(modelViewMatrix, projectionMatrix);

}

void SetupRC()
{
    glClearColor(0.0f, 0.0f, 0.0f, 0.0f);
    shaderManager.InitializeStockShaders();

    glEnable(GL_DEPTH_TEST);

    glEnable(GL_ALPHA_TEST);
    glAlphaFunc(GL_GREATER, 0.5f);

    const char* filePath = "/Users/WensarHobby/Documents/Codes/workspace/Volume_Rendering/Volume_Rendering/head256x256x109";
    if(!InitTextures2D(filePath))
    {
        printf("InitTexture error");
    }
}

bool InitTextures2D(const char* filePath)
{
    std::fstream myFile;
    myFile.open(filePath, std::ios::in | std::ios::binary);

    m_uImageCount = 109;
    m_uImageWidth = 256;
    m_uImageHeight = 256;

    // Holds the texuture IDs
    m_puTextureIDs = new GLuint[m_uImageCount];

    // Holds the luminance buffer
    char* chBuffer = new char[m_uImageWidth * m_uImageHeight];
    char* chRGBABuffer = new char[m_uImageWidth * m_uImageHeight * 4];
    glGenTextures(m_uImageCount, m_puTextureIDs);

    // Read each frames and construct the texture
    for( int nIndx = 0; nIndx < m_uImageCount; ++nIndx )
    {
        // Read the frame
        myFile.read(chBuffer, m_uImageWidth*m_uImageHeight);



        // Convert the data to RGBA data.
        // Here we are simply putting the same value to R, G, B and A channels.
        // Usually for raw data, the alpha value will
        // be constructed by a threshold value given by the user

        for( int nIndx = 0; nIndx < m_uImageWidth*m_uImageHeight; ++nIndx )
        {
            chRGBABuffer[nIndx*4] = chBuffer[nIndx];
            chRGBABuffer[nIndx*4+1] = chBuffer[nIndx];
            chRGBABuffer[nIndx*4+2] = chBuffer[nIndx];
            //printf("%i  ", chBuffer[nIndx]);
            if( chBuffer[nIndx] < 20 )
            {
                chRGBABuffer[nIndx*4+3] = 0;
            }
            else
            {
                chRGBABuffer[nIndx*4+3] = 255;
            }
        }


        // Set the properties of the texture.
        glBindTexture( GL_TEXTURE_2D, m_puTextureIDs[nIndx] );
        glTexEnvi(GL_TEXTURE_ENV, GL_TEXTURE_ENV_MODE, GL_REPLACE);
        glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_WRAP_S, GL_CLAMP);
        glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_WRAP_T, GL_CLAMP);
        glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_MAG_FILTER, GL_LINEAR);
        glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_MIN_FILTER, GL_LINEAR);

        glTexImage2D(GL_TEXTURE_2D, 0, GL_RGBA, m_uImageWidth, m_uImageHeight , 0,
                 GL_RGBA, GL_UNSIGNED_BYTE,(GLvoid *) chRGBABuffer);
        glBindTexture( GL_TEXTURE_2D, 0 );
    }

    delete[] chBuffer;
    delete[] chRGBABuffer;
    return true;
}

void SpecialKeys(int key, int x, int y)
{
    glutPostRedisplay();
}

void RenderScene(void)
{
    static GLfloat vLightPos [] = { 1.0f, 1.0f, 0.0f };
    static GLfloat vWhite [] = { 1.0f, 1.0f, 1.0f, 1.0f };

    glClear(GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT | GL_STENCIL_BUFFER_BIT);


    modelViewMatrix.PushMatrix();
    M3DMatrix44f mCamera;
    cameraFrame.GetCameraMatrix(mCamera);
    modelViewMatrix.MultMatrix(mCamera);

//    M3DMatrix44f mObjectFrame;
//    objectFrame.GetMatrix(mObjectFrame);
//    modelViewMatrix.MultMatrix(mObjectFrame);

    for(int nIndx=m_uImageCount - 1; nIndx >= 0;nIndx--)
    {
        glBindTexture(GL_TEXTURE_2D, m_puTextureIDs[nIndx]);
        MakeQuads(nIndx);
        glBindTexture(GL_TEXTURE_2D, m_puTextureIDs[nIndx]);
        shaderManager.UseStockShader(GLT_SHADER_TEXTURE_POINT_LIGHT_DIFF,
                                 transformPipeline.GetModelViewMatrix(),
                                 transformPipeline.GetProjectionMatrix(),
                                 vLightPos, vWhite, 0);
        myBatch.Draw();
        myBatch.Reset();
    }

    modelViewMatrix.PopMatrix();

    glutSwapBuffers();

}

void MakeQuads(int quads_index)
{
    myBatch.Begin(GL_QUADS, 4, 1);

    myBatch.Normal3f(0.0f, 0.0f, -1.0f);
    myBatch.MultiTexCoord2f(0, 0.0f, 0.0f);
    myBatch.Vertex3f(-1.0f, -1.0f, 1.0f - 2.0f * (GLfloat)(quads_index/m_uImageCount));

    myBatch.Normal3f(0.0f, 0.0f, -1.0f);
    myBatch.MultiTexCoord2f(0, 1.0f, 0.0f);
    myBatch.Vertex3f(1.0f, -1.0f, 1.0f - 2.0f * (GLfloat)(quads_index/m_uImageCount));

    myBatch.Normal3f(0.0f, 0.0f, -1.0f);
    myBatch.MultiTexCoord2f(0, 1.0f, 1.0f);
    myBatch.Vertex3f(1.0f, 1.0f, 1.0f - 2.0f * (GLfloat)(quads_index/m_uImageCount));

    myBatch.Normal3f(0.0f, 0.0f, -1.0f);
    myBatch.MultiTexCoord2f(0, 0.0f, 1.0f);
    myBatch.Vertex3f(-1.0f, 1.0f, 1.0f - 2.0f * (GLfloat)(quads_index/m_uImageCount));

    myBatch.End();
}

void ShutdownRC(void)
{
    glDeleteTextures(m_uImageCount, (GLuint*)m_puTextureIDs);
}


int main(int argc, char* argv[])
{
    gltSetWorkingDirectory(argv[0]);
    glutInit(&argc, argv);
    glutInitDisplayMode(GLUT_DOUBLE | GLUT_RGBA | GLUT_DEPTH | GLUT_STENCIL);
    glutInitWindowSize(400, 400);
    glutCreateWindow("Volume_Rendering");
    glutReshapeFunc(ChangeSize);
    glutSpecialFunc(SpecialKeys);
    glutDisplayFunc(RenderScene);

    GLenum err = glewInit();
    if (GLEW_OK != err) {
        fprintf(stderr, "GLEW Error: %s\n", glewGetErrorString(err));
        return 1;
    }

    SetupRC();

    glutMainLoop();

    ShutdownRC();
    return 0;

}

如您所见，在InitTextures2D函数中，我将原始数据转换为RGBA数据。如果一个像素的亮度值低于20，我假设它是黑色并将α值设置为0（透明）。对于其他像素，alpha值都设置为255（不透明）。所以在我看来，由于像素的alpha值可以是0或255，所以无论我赋予函数glAlphaFunc（GL_GREATER，reference_value）的第二个参数的值是什么，最终结果的外观应该总是相同的。我做了一些测试但不幸的是结果完全不同。

glAlphaFunc（GL_GREATER，0.1f）的结果： Result of glAlphaFunc(GL_GREATER, 0.1f) glAlphaFunc（GL_GREATER，0.5f）的结果： glAlphaFunc的结果（GL_GREATER，0.99f）：

告诉我原因。

Answer 1

所以在我看来，因为像素的alpha值可以是0或255

这不完全正确。真实的是 texels 的alpha值将为0或255.但是，您使用的是GL_LINEAR图像过滤器，并且也不会将您的纹素映射到输出像素1：1，所以你实际上在一些纹素之间进行采样，并且当在透明＆＃34;之间采样时可以获得0到255之间的任何值。和＆＃34;不透明＆＃34;边界。

OpenGL alpha功能

1 个答案: