Using single channel texture (OpenGL 2)? - c++

Question

Short storry:
when I render anything using texture loaded like this
glTexImage2D ( GL_TEXTURE_2D, 0, GL_R8, width, height, 0, GL_RED, GL_UNSIGNED_BYTE, pixels );
I get only black
Long storry:
I can get RGBA texture with alpha channel (e.g. text with transparent backgorund using this code):
This code works:
// === load
#define GL_ABGR 0x8000
SDL_Surface * surf = SDL_LoadBMP( "common_resources/dejvu_sans_mono_RGBA.bmp" );
glGenTextures ( 1, &itex );
glBindTexture ( GL_TEXTURE_2D, itex );
glTexImage2D ( GL_TEXTURE_2D, 0, GL_RGBA, surf->w, surf->h, 0, GL_ABGR, GL_UNSIGNED_BYTE, surf->pixels );
glTexParameteri( GL_TEXTURE_2D, GL_TEXTURE_MIN_FILTER, GL_LINEAR );
glTexParameteri( GL_TEXTURE_2D, GL_TEXTURE_MAG_FILTER, GL_LINEAR );
// ....
// === render
glEnable( GL_TEXTURE_2D );
glBindTexture( GL_TEXTURE_2D, itex );
glColor3f(1.0f,1.0f,1.0f);
glEnable(GL_BLEND);
glEnable(GL_ALPHA_TEST);
glBlendFunc(GL_SRC_ALPHA,GL_ONE_MINUS_SRC_ALPHA);
drawString ( caption, xmin, ymin+12, 6 );
renders like
But I'm trying to use just one channel (8-bit; grayscale) images / textures instead of RGBA. These I cannot get to render neither with nor without transparancy. Whatever I do I get only black image.
This doesn't
// === load
#define GL_ABGR 0x8000
SDL_Surface * surf = SDL_LoadBMP( "common_resources/dejvu_sans_mono_Alpha.bmp" );
glGenTextures ( 1, &itex );
glBindTexture ( GL_TEXTURE_2D, itex );
glTexImage2D ( GL_TEXTURE_2D, 0, GL_R8, surf->w, surf->h, 0, GL_RED, GL_UNSIGNED_BYTE, surf->pixels );
glTexParameteri( GL_TEXTURE_2D, GL_TEXTURE_MIN_FILTER, GL_LINEAR );
glTexParameteri( GL_TEXTURE_2D, GL_TEXTURE_MAG_FILTER, GL_LINEAR );
// ....
// === render
glEnable( GL_TEXTURE_2D );
glBindTexture( GL_TEXTURE_2D, itex );
glColor3f(1.0f,1.0f,1.0f);
//glEnable(GL_BLEND);
//glEnable(GL_ALPHA_TEST);
//glBlendFunc(GL_SRC_ALPHA,GL_ONE_MINUS_SRC_ALPHA);
drawString ( caption, xmin, ymin+12, 6 );
renders like
Notes:
I know that I should somehow use glTexEnv according to e.g. here but my main problem is that apparently the monochrome texture does not render at all
I tried also other GL_LUMINANCE and GL_INTENSITY instead of GL_RED in glTexImage2D with no difference
there are other questions like here and here but mostly with OpenGL>3.0 and fragment shaders
Also, is it possible that my graphics card or driver does not support this ? I'm on ubuntu 16.04
GL_VENDOR: Intel Open Source Technology Center
GL_RENDERER: Mesa DRI Intel(R) HD Graphics 530 (Skylake GT2)
GL_VERSION: 3.0 Mesa 11.2.0
for completeness - although it is not importaint the drawString looks like this:
drawString ( caption, xmin, ymin+12, 6 ){
const int nchars = 95;
float persprite = 1.0f/nchars;
glBegin(GL_QUADS);
for(int i=0; i<65536; i++){
if( str[i] == 0 ) break; // 0-terminated string
int isprite = str[i] - 33; // 33 is offset of meaningfull ASCII characters
float offset = isprite*persprite+(persprite*0.57);
float xi = i*sz + x;
glTexCoord2f( offset , 1.0f ); glVertex3f( xi, y, 3.0f );
glTexCoord2f( offset+persprite, 1.0f ); glVertex3f( xi+sz, y, 3.0f );
glTexCoord2f( offset+persprite, 0.0f ); glVertex3f( xi+sz, y+sz*2, 3.0f );
glTexCoord2f( offset , 0.0f ); glVertex3f( xi, y+sz*2, 3.0f );
}
glEnd();
}

Answer 1

I want to try to help you. In my projects I am using this arguments for generating textures from grayscale source images:
glTexImage2D(GL_TEXTURE_2D, 0, 1, width, height, 0, GL_RED,
GL_UNSIGNED_BYTE, pixels);
As written in documentation, third argument - number of color components (1 in our case). Need to check integer value of GL_R8 or replace it explicitly.
GL_RED means that you place luminances in red channel (not in each red, green, blue channels as for grayscale image).