用glReadPixels读取纹理字节？

Question

我希望将原始纹理数据转储到磁盘（以便以后读取），但我不确定glReadPixel是否会从当前绑定的纹理中读取。

我该如何从我的纹理中读取缓冲区？

- Geri Borbás

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嘿 @Geri，我的回答对你有帮助吗？ - gergonzalez

Answer 1

glReadPixels函数从帧缓冲区读取数据，而不是纹理。如果要读取纹理对象，则必须使用glGetTexImage，但OpenGL ES不支持该函数 :(

如果您想从纹理中读取缓冲区，则可以将其绑定到FBO（FrameBuffer Object）并使用glReadPixels：

//Generate a new FBO. It will contain your texture.
glGenFramebuffersOES(1, &offscreen_framebuffer);
glBindFramebufferOES(GL_FRAMEBUFFER_OES, offscreen_framebuffer);

//Create the texture 
glGenTextures(1, &my_texture);
glBindTexture(GL_TEXTURE_2D, my_texture);
glTexImage2D(GL_TEXTURE_2D, 0, GL_RGBA,  width, height, 0, GL_RGBA, GL_UNSIGNED_BYTE, NULL);
glTexParameteri(GL_TEXTURE_2D,GL_TEXTURE_MIN_FILTER,GL_LINEAR);
glTexParameteri(GL_TEXTURE_2D,GL_TEXTURE_MAG_FILTER,GL_LINEAR);

//Bind the texture to your FBO
glFramebufferTexture2DOES(GL_FRAMEBUFFER_OES, GL_COLOR_ATTACHMENT0_OES, GL_TEXTURE_2D, my_texture, 0);

//Test if everything failed    
GLenum status = glCheckFramebufferStatusOES(GL_FRAMEBUFFER_OES);
if(status != GL_FRAMEBUFFER_COMPLETE_OES) {
    printf("failed to make complete framebuffer object %x", status);
}

接下来，只需要在需要从纹理中读取数据时调用glReadPixels函数：

//Bind the FBO
glBindFramebufferOES(GL_FRAMEBUFFER_OES, offscreen_framebuffer);
// set the viewport as the FBO won't be the same dimension as the screen
glViewport(0, 0, width, height);

GLubyte* pixels = (GLubyte*) malloc(width * height * sizeof(GLubyte) * 4);
glReadPixels(0, 0, width, height, GL_RGBA, GL_UNSIGNED_BYTE, pixels);

//Bind your main FBO again
glBindFramebufferOES(GL_FRAMEBUFFER_OES, screen_framebuffer);
// set the viewport as the FBO won't be the same dimension as the screen
glViewport(0, 0, screen_width, screen_height);

- gergonzalez

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一旦移除了所有的OES后缀，我可以确认这也适用于Mac操作系统。 - nuclearnova

在开始时，使用GLuint screen_framebuffer; glGetIntegerv(GL_FRAMEBUFFER_BINDING, ＆screen_framebuffer);获取主FBO id。 - Matthew Grivich

Answer 2

感谢Gergonzale的回答。今天早上我花了一些时间尝试弄清楚如何将16位纹理转换为GL_UNSIGNED_BYTE，这段代码可能对其他人有用。

glTexImage2D(GL_TEXTURE_2D, 0, GL_RGB,  tSizeW, tSizeH, 0, GL_RGB, GL_UNSIGNED_SHORT_5_6_5, NULL);
GLubyte* pixels = (GLubyte*) malloc(tSizeW * tSizeH * sizeof(GLubyte) * 2);
glReadPixels(0, 0, tSizeW, tSizeH, GL_RGB, GL_UNSIGNED_SHORT_5_6_5, pixels);

int index = (x*tSizeH + y)*2;
unsigned int rgb = pixels[index + 1]*256 + pixels[index + 0];
unsigned int r = rgb;
r &= 0xF800;    // 1111 1000 0000 0000
r >>= 11;       // 0001 1111
r *= (255/31.); // Convert from 31 max to 255 max

unsigned int g = rgb;
g &= 0x7E0;     // 0000 0111 1110 0000
g >>= 5;        // 0011 1111
g *= (255/63.); // Convert from 63 max to 255 max

unsigned int b = rgb;
b &= 0x1F;      // 0000 0000 0001 1111
//g >>= 0;      // 0001 1111
b *= (255/31.); // Convert from 31 max to 255 max

- nuclearnova