Scale OpenGL texture and return bitmap in CPU memory - macos

Question

I have a texture on the GPU defined by an OpenGL textureID and target.
I need for further processing a 300 pixel bitmap in CPU memory (width 300 pixel, height proportional depending on the source width).
The pixel format should be RGBA, ARGB or BGRA with float components.
How can this be done?
Thanks for your reply.
I tried the following. But I get only white pixels back:
glEnable(GL_TEXTURE_2D);
// create render texture
GLuint renderedTexture;
glGenTextures(1, &renderedTexture);
glBindTexture(GL_TEXTURE_2D, renderedTexture);
glTexImage2D(GL_TEXTURE_2D, 0,GL_RGB, (GLsizei)analyzeWidth, (GLsizei)analyzeHeight, 0,GL_RGBA, GL_FLOAT, 0);
unsigned int fbo;
glGenFramebuffers(1, &fbo);
glBindFramebuffer(GL_FRAMEBUFFER, fbo);
glFramebufferTexture2D(GL_FRAMEBUFFER, GL_COLOR_ATTACHMENT0, GL_TEXTURE_2D, renderedTexture, 0);
// draw texture
glBindTexture(GL_TEXTURE_2D, inTextureId);
glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_MAG_FILTER, GL_LINEAR);
glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_MIN_FILTER, GL_LINEAR);
// Draw a textured quad
glBegin(GL_QUADS);
glTexCoord2f(0, 0); glVertex3f(0, 0, 0);
glTexCoord2f(0, 1); glVertex3f(0, 1, 0);
glTexCoord2f(1, 1); glVertex3f(1, 1, 0);
glTexCoord2f(1, 0); glVertex3f(1, 0, 0);
glEnd();
GLenum status = glCheckFramebufferStatus(GL_FRAMEBUFFER);
if(status == GL_FRAMEBUFFER_COMPLETE)
{
}
unsigned char *buffer = CGBitmapContextGetData(mainCtx);
glReadPixels(0, 0, (GLsizei)analyzeWidth, (GLsizei)analyzeHeight, GL_RGBA, GL_FLOAT, buffer);
glDisable(GL_TEXTURE_2D);
glBindFramebuffer(GL_FRAMEBUFFER, 0); //
glDeleteFramebuffers(1, &fbo);

Answer 1

Create a second texture with the size 300*width/height x 300
Create a Framebuffer Object and attach the new texture as color buffer.
Set approrpiate texture filters for the (unscaled) source texture. You have the choice between point sampling (GL_NEAREST) and bilinear filtering (GL_LINEAR). If you are downscaling by more than a factor of 2 you might consider also using mipmapping, and might want to call glGenerateMipmap on the source texture first, and use one of the GL_..._MIPMAP_... minification filters. However, the availability of mipmapping will depend on how the source texture was created, if it is an immutable texture object without the mipmap pyramid, this won't work.
Render a textured object (with the original source texture) to the new texture. Most intuitive geometry would be a viewport-filling rectangle, most efficient would be a single triangle.
Read back the scaled texture with glReadPixels (via the FBO) or glGetTexImage (directly from the texture). For improved performance, you might consider asynchronous readbacks via Pixel Buffer Objects.