ios - 用 glReadPixels 读取纹理字节？

Question

我想将原始纹理数据转储到磁盘（稍后再读），我不确定 glReadPixel 是否会从当前绑定的纹理中读取。

如何从纹理中读取缓冲区？

Answer 1

glReadPixels函数从帧缓冲区读取，而不是纹理。要读取纹理对象，您必须使用glGetTexImage但它在 OpenGL ES 中不可用:(

如果要从纹理中读取缓冲区，则可以将其绑定到 FBO（FrameBuffer 对象）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);
}

然后，当您想从纹理中读取数据时，您只需调用 glReadPixels ：

//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);

Answer 2

感谢 Gergonzale 的回答。我今天早上花了一些时间试图弄清楚如何让它与 16 位纹理一起工作，这个代码片段可能对任何其他将 GL_UNSIGNED_SHORT_5_6_5 转换为 GL_UNSIGNED_BYTE 的人有用

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