Puzzels about glteximage2d using gl_luminance

I met some problem about using gl_luminance to define FBO. Here is the code i used,

 generateRenderToTexture(GL_RGBA, GL_RGBA, GL_UNSIGNED_BYTE, _maskTexture, _imageWidth, _imageHeight, false);

related code is as follows,

TextureBuffer _maskTexture;

class TextureBuffer {
public:
GLuint texture;
GLuint frameBuffer;
GLenum internalformat;
GLenum format;
GLenum type;
int    w,h;

TextureBuffer() : texture(0), frameBuffer(0) {}
void release() 
{
    if(texture)
    {
        glDeleteTextures(1, &texture);
        texture = 0;
    }
    if(frameBuffer)
    {
        glDeleteFramebuffers(1, &frameBuffer);
        frameBuffer = 0;
    }

}
};

void generateRenderToTexture(GLint internalformat, GLenum format, GLenum type,
                                     TextureBuffer &tb, int w, int h, bool linearInterp)
{
    glGenTextures(1, &tb.texture);
    glBindTexture(GL_TEXTURE_2D, tb.texture);
    glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_WRAP_S, GL_CLAMP_TO_EDGE);
    glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_WRAP_T, GL_CLAMP_TO_EDGE);
    glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_MIN_FILTER, linearInterp ? GL_LINEAR : GL_NEAREST);
    glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_MAG_FILTER, linearInterp ? GL_LINEAR : GL_NEAREST);
    glTexImage2D(GL_TEXTURE_2D, 0, internalformat, w, h, 0, format, type, NULL);

    glGenFramebuffers(1, &tb.frameBuffer);
    glBindFramebuffer(GL_FRAMEBUFFER, tb.frameBuffer);
    glClear(_glClearBits);
    glFramebufferTexture2D(GL_FRAMEBUFFER, GL_COLOR_ATTACHMENT0, GL_TEXTURE_2D, tb.texture, 0);

    GLenum status = glCheckFramebufferStatus(GL_FRAMEBUFFER);
    if(status != GL_FRAMEBUFFER_COMPLETE)
        printf("Framebuffer status: %x", (int)status);

    tb.internalformat = internalformat;
    tb.format = format;
    tb.type = type;
    tb.w = w;
    tb.h = h;
}

The question is when I use,

generateRenderToTexture(GL_RGBA, GL_RGBA, GL_UNSIGNED_BYTE, _maskTexture, _imageWidth, _imageHeight, false);

The code went well. But if use gl_luminance instead,

generateRenderToTexture(GL_LUMINANCE, GL_LUMINANCE, GL_UNSIGNED_BYTE,  _maskTexture,  _imageWidthOriginal, 

I don't know why i could not use GL_LUMINANCE to define the FBO. Anyone have some useful suggestions to solve this?

The only formats that are guaranteed to work as color FBO attachments in ES 2.0 are, according to table 4.5 in the spec document:

• GL_RGBA4
• GL_RGB5_A1
• GL_RGB565

Support for rendering to GL_RGBA, which works for you, is not required by the standard. Many implementations support it, though. The OES_rgb8_rgba8 extension adds support for GL_RGB8 and GL_RGBA8 formats as render targets.

GL_LUMINANCE is not supported as a color-renderable format by the standard, and I can't find an extension for it either. It's possible that some implementations could support it, but you certainly can't count on it.

ES 3.0 lists GL_R8 as a color-renderable format. In ES 3.0, the RED/R formats replace the LUMINANCE/ALPHA formats from ES 2.0. So if you can move to ES 3.0, you have support to render to 1-component texture formats.