How to directly input RGB(A) values in textures in C++/OpenGL? - c++

I want to dabble in some procedural textures in an OpenGL project, but nothing seems to work. All I need is to input the RGB and maybe A values into an empty texture, instead of loading it from a file. How do I do this, in actual, practical code?
Edit: There is no main code yet, because I have found nothing that works. The current best guess is this:
void GenerateTexture()
{
unsigned char image_data[16] = {0, 0, 150, 255, 125, 0, 0, 255, 0, 0, 150, 255, 125, 0, 0, 255};
glGenTextures(1, &tex_obj);
glBindTexture(GL_TEXTURE_2D, tex_obj);
glTexImage2D(GL_TEXTURE_2D, 0, GL_RGBA8, 2, 2, 0, GL_RGBA, GL_UNSIGNED_BYTE, image_data);
}
... implemented like this:
glBegin (GL_QUADS);
if (*irrelevant for question*){
glTexCoord2f(0.0,1.0);
glVertex3f (-0.45-cam.pos.val[0], 0.45-cam.pos.val[1], 0.45-cam.pos.val[2]);
glTexCoord2f(0.0,0.0);
glVertex3f (-0.45-cam.pos.val[0], -0.45-cam.pos.val[1], 0.45-cam.pos.val[2]);
glTexCoord2f(1.0,0.0);
glVertex3f (0.45-cam.pos.val[0], -0.45-cam.pos.val[1], 0.45-cam.pos.val[2]);
glTexCoord2f(1.0,1.0);
glVertex3f (0.45-cam.pos.val[0], 0.45-cam.pos.val[1], 0.45-cam.pos.val[2]);
}
glEnd ();
... but as soon as it is run with more than a 1x1 texture, it just goes clean white.

Create an array of data and load it into a texture image:
uint8_t image_data[4] = {255, 0, 0, 255}; // red
GLuint tex_obj;
glGenTextures(1, &tex_obj);
glBindTexture(GL_TEXTURE_2D, tex_obj);
glTexImage2D(GL_TEXTURE_2D, 0, GL_RGBA8, 1, 1, 0, GL_RGBA, GL_UNSIGNED_BYTE, image_data);

Related

Why doesn't glBlitNamedFramebuffer() work properly?

I am playing a bit with bit blitting in OpenGL. When I run the code looking like this:
GLuint fbo = CreateFBO(); // creates fbo and attaches a texture to it
glBindFramebuffer(GL_FRAMEBUFFER, 0);
GLfloat some_color[] = {0.8, 0.4, 0.6, 1.0};
glClearBufferfv(GL_COLOR, 0, some_color);
glBlitNamedFramebuffer(0, fbo, 0, 0, 100, 100, 0, 0, 100, 100, GL_COLOR_BUFFER_BIT, GL_NEAREST);
glClear(GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT |GL_STENCIL_BUFFER_BIT);
glBlitNamedFramebuffer(fbo, 0, 0, 0, 100, 100, 0, 0, 100, 100, GL_COLOR_BUFFER_BIT, GL_NEAREST);
It yields a black screen, that is, the screen was cleared, although it should have the color of some_color. But if I change the "named" function to the "unnamed" counterpart, then it works as intended. The new piece of code looks like:
GLuint fbo = CreateFBO(); // creates fbo and attaches a texture to it
glBindFramebuffer(GL_FRAMEBUFFER, 0);
GLfloat some_color[] = {0.8, 0.4, 0.6, 1.0};
glClearBufferfv(GL_COLOR, 0, some_color);
glBindFramebuffer(GL_READ_FRAMEBUFFER, 0);
glBindFramebuffer(GL_DRAW_FRAMEBUFFER, fbo_);
glBlitFramebuffer(0, 0, 100, 100, 0, 0, 100, 100, GL_COLOR_BUFFER_BIT, GL_NEAREST);
glBindFramebuffer(GL_FRAMEBUFFER, 0);
glClear(GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT |GL_STENCIL_BUFFER_BIT);
glBindFramebuffer(GL_READ_FRAMEBUFFER, fbo_);
glBindFramebuffer(GL_DRAW_FRAMEBUFFER, 0);
glBlitFramebuffer(0, 0, 100, 100, 0, 0, 100, 100, GL_COLOR_BUFFER_BIT, GL_NEAREST);
And executing this code, I get the expected result, namely the screen having the color of some_color. What am I doing wrong employing bit blitting? Or if I am using it correctly, why doesn't it work properly?
UPD: I mixed up blit functions in the second code sample.
It yields a black screen, that is, the screen was cleared, although it should have the color of some_color.
No it shouldn't. The first parameter to glBlitNamedFramebuffer is the read framebuffer. So your blit reads from the default framebuffer and writes to the FBO. You clear the screen. Then you reverse this operation. This copies from the FBO to the default framebuffer. Thus overwriting the clear.
Whatever it is you see on the screen, it should be anything but some_color.

Unable to apply texture on my cube using opengl & sfml

I want to load a texture on one of my faces on a cube. All the other faces have a specific color that I put on them. The problem is that, when I try to apply the texture, it will not display it. Instead, the last texture used on the cube is being applied.
Here is a code sample :
Engine::Engine() : m_isMovingUp(false), m_isMovingDown(false), m_isMovingLeft(false), m_isMovingRight(false), m_context(24, 8, 4, 3, 3), m_angleX(0), m_angleZ(0), m_mov(3.0f, 3.0f, 3.0f)
{
//Window settings
m_win.create(sf::VideoMode(800, 600), "SFML Event and with some OpenGL graphics", sf::Style::Default, m_context);
m_win.setFramerateLimit(60);
//Opengl settings
glEnable(GL_TEXTURE_2D);
m_textureID = loadTexture("/Data/test.jpg");
}
void Engine::run()
{
bool running = true;
while (running)
{
processEvents();
update();
render();
}
}
void Engine::cube()
{
//glRotated(m_angleX, 1, 0, 0);
//glRotated(m_angleZ, 0, 0, 1);
glMatrixMode(GL_MODELVIEW);
glLoadIdentity();
glBindTexture(GL_TEXTURE_2D, m_textureID);
glBegin(GL_QUADS);
//face Rouge
//glColor3ub(255, 0, 0);
glTexCoord2d(0, 1); glVertex3d(1, 1, 1);
glTexCoord2d(0, 0); glVertex3d(1, 1, -1);
glTexCoord2d(1, 0); glVertex3d(-1, 1, -1);
glTexCoord2d(1, 1); glVertex3d(-1, 1, 1);
glEnd();
glBegin(GL_QUADS);
//face verte
glColor3ub(0, 255, 0);
glVertex3d(1, -1, 1);
glVertex3d(1, -1, -1);
glVertex3d(1, 1, -1);
glVertex3d(1, 1, 1);
//face bleu
glColor3ub(0, 0, 255);
glVertex3d(1, -1, 1);
glVertex3d(-1, -1, 1);
glVertex3d(-1, -1, -1);
glVertex3d(1, -1, -1);
//face jaune
glColor3ub(255, 255, 0);
glVertex3d(-1, 1, 1);
glVertex3d(-1, 1, -1);
glVertex3d(-1, -1, -1);
glVertex3d(-1, -1, 1);
//face cyan
glColor3ub(0, 255, 255);
glVertex3d(1, 1, -1);
glVertex3d(1, -1, -1);
glVertex3d(-1, -1, -1);
glVertex3d(-1, 1, -1);
//face magenta
glColor3ub(255, 0, 255);
glVertex3d(1, -1, 1);
glVertex3d(1, 1, 1);
glVertex3d(-1, 1, 1);
glVertex3d(-1, -1, 1);
glEnd();
glFlush();
}
GLuint Engine::loadTexture(string file)
{
GLuint textureID;
if (!m_image.loadFromFile(file))
EXIT_FAILURE;
glGenTextures(1, &textureID);
//glBindTexture(GL_TEXTURE_2D, textureID);
glTexImage2D(GL_TEXTURE_2D, 0, 4, m_image.getSize().x, m_image.getSize().y, 0, GL_RGBA, GL_UNSIGNED_BYTE, m_image.getPixelsPtr());
glTexParameterf(GL_TEXTURE_2D, GL_TEXTURE_MIN_FILTER, GL_LINEAR);
glTexParameterf(GL_TEXTURE_2D, GL_TEXTURE_MAG_FILTER, GL_LINEAR);
return textureID;
}
This is where I'm at after fixing as much as I could.
PS : CODE UPDATED & PICTURE ADDED
Picture of my cube
I see two mistakes in the pastebin code:
Line 27:
glBindTexture(GL_TEXTURE_2D, 0);
You can not change the bound texture in between glBegin and glEnd calls. You will have to split rendering of the textured and not textured faces to their own begin-end blocks.
Line 74:
glTexImage2D(GL_TEXTURE_2D, 0, 4, m_image.getSize().x, m_image.getSize().y, 0, GL_RGBA, GL_UNSIGNED_BYTE, &m_image.getPixelsPtr);
The last parameter should be m_image.getPixelsPtr() not &m_image.getPixelsPtr. You are taking address of the method and using it as image data. The method getPixelsPtr will give you pointer to the image data if you call it.
Also it seems that you are creating new texture and loading image into it on every frame. If it is just in this example that renders once, then it is ok. But in actual program it would have big impact on performance and the program would soon run out of memory.
There may be more problems, it can't be told without seeing the whole code. If you can, upload somewhere a minimal complete example.

Framebuffer Texture rendered to screen is stretched at certain points

I'm currently trying to test out rendering to a framebuffer for various uses, but whenever I have an object(say a square) at a certain y-value, it appears "stretched", and then past a certain y-value or a certain x-value it seems to "thin out" and disappears. I have determined the x and y-values that it disappears at, but the coordinates seem to not have any rhyme or reason.
When I remove the framebuffer binding and render directly to the screen it draws the square perfectly fine, no matter the x or y-value.
Drawing a basic square(using immediate mode to remove possible errors) with a wide x-value looks like this:
Code here:
Window window("Framebuffer Testing", 1600, 900); //1600x900 right now
int fbowidth = 800, fboheight = 600;
mat4 ortho = mat4::orthographic(0, width, 0, height, -1.0f, 1.0f);
//trimmed out some code from shader creation that is bugless and unneccessary to include
Shader shader("basic"); shader.setUniform("pr_matrix", ortho);
Shader drawFromFBO("fbotest"); shader.setUniform("pr_matrix", ortho);
GLfloat screenVertices[] = {
0, 0, 0, 0, height, 0,
width, height, 0, width, 0, 0};
GLushort indices[] = {
0, 1, 2,
2, 3, 0 };
GLfloat texcoords[] = { //texcoords sent to the drawFromFBO shader
0, 0, 0, 1, 1, 1,
1, 1, 1, 0, 0, 0 };
IndexBuffer ibo(indices, 6);
VertexArray vao;
vao.addBuffer(new Buffer(screenVertices, 4 * 3, 3), 0);
vao.addBuffer(new Buffer(texcoords, 2 * 6, 2), 1);
GLuint fbo;
glGenFramebuffers(1, &fbo);
glBindFramebuffer(GL_FRAMEBUFFER, fbo);
GLuint texture;
glGenTextures(1, &texture);
glBindTexture(GL_TEXTURE_2D, texture);
glTexImage2D(GL_TEXTURE_2D, 0, GL_RGB, fbowidth, fboheight, 0, GL_RGB, GL_UNSIGNED_BYTE, NULL);
glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_MAG_FILTER, GL_NEAREST);
glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_MIN_FILTER, GL_NEAREST);
glBindTexture(GL_TEXTURE_2D, 0);
glFramebufferTexture(GL_FRAMEBUFFER, GL_COLOR_ATTACHMENT0, texture, 0);
if(glCheckFramebufferStatus(GL_FRAMEBUFFER) != GL_FRAMEBUFFER_COMPLETE)
std::cout << "false" << std::endl;
glEnable(GL_TEXTURE_2D);
//the x-values mess up at ~783 thru 800 and the y-values at 0 thru ~313
while(!window.closed()) {
glClearColor(0.2f, 0.2f, 0.2f, 1.0f); //grey
window.clear(); //calls glclear for depth and color buffer
//bind framebuffer and shader
shader.enable(); //literally just calls glUseProgram(id) with the compiled shader id
glViewport(0, 0, fbowidth, fboheight);
glBindFramebuffer(GL_FRAMEBUFFER, fbo); //bind the fbo
glClearColor(1.0f, 0.0f, 1.0f, 1.0f); //set clear color to pink
glClear(GL_COLOR_BUFFER_BIT);
//render a red square to the framebuffer texture
glBegin(GL_QUADS); {
glColor3f(1.0f, 0.0f, 0.0f); //set the color to red
glVertex3f(700, 400, 0);
glVertex3f(700, 450, 0);
glVertex3f(750, 450, 0);
glVertex3f(750, 400, 0);
} glEnd();
shader.disable();
glBindFramebuffer(GL_FRAMEBUFFER, 0); //set framebuffer to the default
//render from framebuffer to screen
glViewport(0, 0, width, height);
drawFromFBO.enable();
glActiveTexture(GL_TEXTURE0);
drawFromFBO.setUniform1i("texfbo0", 0);
glBindTexture(GL_TEXTURE_2D, texture);
vao.bind();
ibo.bind();
glDrawElements(GL_TRIANGLES, ibo.getCount(), GL_UNSIGNED_SHORT, NULL);
ibo.unbind();
vao.unbind();
drawFromFBO.disable();
window.update();
}
If you want to see any thing extra, the file is located at my Github: here

What are the steps necessary to render my scene to a Framebuffer Object(FBO) and then render that FBO to the screen?

I've got a fairly complicated scene with many GL_POINTS that I need to render. The scene will be largely static, so I'd like to render it to a Framebuffer Object and then only update that FBO when my scene actually changes. I'd then like to render the FBO to the screen each frame.
I've found examples that render an FBO into a texture. I've found examples that render an FBO into a RenderBuffer (still not quite sure what that is). I'm not sure what the steps are to achieve this. Do I need to render to a texture and the draw the texture to the screen?
Can you please enumerate the steps (ideally even in pseudocode or actual code) to render my scene to an FBO and then draw that FBO to the screen.
draw() is sufficient for a placeholder for my own drawing functions.
I provide a minimal FBO example just for this
Basically the steps are: Create FBO with depth renderbuffer and color texture attachment. To render to FBO unbind the target texture, bind FBO, render to FBO. Unbind FBO, bind texture, render.
#include <GL/glew.h>
#include <GL/glut.h>
#include <stdlib.h>
#include <stdio.h>
#include <math.h>
void init();
void display();
int const fbo_width = 512;
int const fbo_height = 512;
GLuint fb, color, depth;
int main(int argc, char *argv[])
{
glutInit(&argc, argv);
glutInitDisplayMode( GLUT_RGBA | GLUT_DOUBLE | GLUT_DEPTH );
glutCreateWindow("FBO test");
glutDisplayFunc(display);
glutIdleFunc(glutPostRedisplay);
glewInit();
init();
glutMainLoop();
return 0;
}
void CHECK_FRAMEBUFFER_STATUS()
{
GLenum status;
status = glCheckFramebufferStatus(GL_DRAW_FRAMEBUFFER);
switch(status) {
case GL_FRAMEBUFFER_COMPLETE:
break;
case GL_FRAMEBUFFER_UNSUPPORTED:
/* choose different formats */
break;
default:
/* programming error; will fail on all hardware */
fputs("Framebuffer Error\n", stderr);
exit(-1);
}
}
float const light_dir[]={1,1,1,0};
float const light_color[]={1,0.95,0.9,1};
void init()
{
glGenFramebuffers(1, &fb);
glGenTextures(1, &color);
glGenRenderbuffers(1, &depth);
glBindFramebuffer(GL_FRAMEBUFFER, fb);
glBindTexture(GL_TEXTURE_2D, color);
glTexImage2D( GL_TEXTURE_2D,
0,
GL_RGBA,
fbo_width, fbo_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);
glFramebufferTexture2D(GL_DRAW_FRAMEBUFFER, GL_COLOR_ATTACHMENT0, GL_TEXTURE_2D, color, 0);
glBindRenderbuffer(GL_RENDERBUFFER, depth);
glRenderbufferStorage(GL_RENDERBUFFER, GL_DEPTH_COMPONENT24, fbo_width, fbo_height);
glFramebufferRenderbuffer(GL_DRAW_FRAMEBUFFER, GL_DEPTH_ATTACHMENT, GL_RENDERBUFFER, depth);
CHECK_FRAMEBUFFER_STATUS();
}
void prepare()
{
static float a=0, b=0, c=0;
glBindTexture(GL_TEXTURE_2D, 0);
glEnable(GL_TEXTURE_2D);
glBindFramebuffer(GL_FRAMEBUFFER, fb);
glViewport(0,0, fbo_width, fbo_height);
glClearColor(1,1,1,0);
glClear( GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT );
glMatrixMode(GL_PROJECTION);
glLoadIdentity();
gluPerspective(45, 1, 1, 10);
glMatrixMode(GL_MODELVIEW);
glLoadIdentity();
glEnable(GL_LIGHT0);
glEnable(GL_LIGHTING);
glEnable(GL_DEPTH_TEST);
glDisable(GL_CULL_FACE);
glLightfv(GL_LIGHT0, GL_POSITION, light_dir);
glLightfv(GL_LIGHT0, GL_DIFFUSE, light_color);
glTranslatef(0,0,-5);
glRotatef(a, 1, 0, 0);
glRotatef(b, 0, 1, 0);
glRotatef(c, 0, 0, 1);
glutSolidTeapot(0.75);
a=fmod(a+0.1, 360.);
b=fmod(b+0.5, 360.);
c=fmod(c+0.25, 360.);
}
void final()
{
static float a=0, b=0, c=0;
const int win_width = glutGet(GLUT_WINDOW_WIDTH);
const int win_height = glutGet(GLUT_WINDOW_HEIGHT);
const float aspect = (float)win_width/(float)win_height;
glBindFramebuffer(GL_FRAMEBUFFER, 0);
glViewport(0,0, win_width, win_height);
glClearColor(1.,1.,1.,0.);
glClear( GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT );
glMatrixMode(GL_PROJECTION);
glLoadIdentity();
gluPerspective(45, aspect, 1, 10);
glMatrixMode(GL_MODELVIEW);
glLoadIdentity();
glTranslatef(0,0,-5);
glRotatef(b, 0, 1, 0);
b=fmod(b+0.5, 360.);
glEnable(GL_TEXTURE_2D);
glBindTexture(GL_TEXTURE_2D, color);
glEnable(GL_DEPTH_TEST);
glEnable(GL_CULL_FACE);
glEnable(GL_BLEND);
glBlendFunc(GL_SRC_ALPHA, GL_ONE_MINUS_SRC_ALPHA);
glDisable(GL_LIGHTING);
float cube[][5]=
{
{-1, -1, -1, 0, 0},
{ 1, -1, -1, 1, 0},
{ 1, 1, -1, 1, 1},
{-1, 1, -1, 0, 1},
{-1, -1, 1, -1, 0},
{ 1, -1, 1, 0, 0},
{ 1, 1, 1, 0, 1},
{-1, 1, 1, -1, 1},
};
unsigned int faces[]=
{
0, 1, 2, 3,
1, 5, 6, 2,
5, 4, 7, 6,
4, 0, 3, 7,
3, 2, 6, 7,
4, 5, 1, 0
};
glEnableClientState(GL_VERTEX_ARRAY);
glEnableClientState(GL_TEXTURE_COORD_ARRAY);
glVertexPointer(3, GL_FLOAT, 5*sizeof(float), &cube[0][0]);
glTexCoordPointer(2, GL_FLOAT, 5*sizeof(float), &cube[0][3]);
glCullFace(GL_BACK);
glDrawElements(GL_QUADS, 24, GL_UNSIGNED_INT, faces);
glCullFace(GL_FRONT);
glDrawElements(GL_QUADS, 24, GL_UNSIGNED_INT, faces);
glDisableClientState(GL_VERTEX_ARRAY);
glDisableClientState(GL_TEXTURE_COORD_ARRAY);
}
void display()
{
prepare();
final();
glutSwapBuffers();
}
Here is alternative example which does not require textures:
// copy framebuffer
if(fboUsed)
{
glBindFramebuffer(GL_READ_FRAMEBUFFER, fboId);
glBindFramebuffer(GL_DRAW_FRAMEBUFFER, 0);
glBlitFramebuffer(0, 0, TEXTURE_WIDTH, TEXTURE_HEIGHT,
0, 0, screenWidth, screenHeight,
GL_COLOR_BUFFER_BIT,
GL_LINEAR);
glBindFramebuffer(GL_READ_FRAMEBUFFER, 0);
}
Replace variables in blit with your own.
Apperently frame buffer 0 is front buffer.
fboId is your frame buffer number.

Why is drawing an 8bit texture with OpenGL drawing black pixels instead of transparent?

OpenGL Setup:
glEnable( GL_BLEND );
glBlendFunc( GL_SRC_ALPHA, GL_ONE_MINUS_SRC_ALPHA );
glEnable( GL_DEPTH_TEST );
glEnable( GL_TEXTURE_2D );
glEnable( GL_CULL_FACE );
glCullFace( GL_BACK );
glClearColor( 0.0, 1.0, 1.0, 1.0 );
Initializing the texture:
// 16x16 X pattern
uint8_t buffer[ 16*16 ] =
{
255, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 255,
0, 255, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 255, 0,
0, 0, 255, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 255, 0, 0,
0, 0, 0, 255, 0, 0, 0, 0, 0, 0, 0, 0, 255, 0, 0, 0,
0, 0, 0, 0, 255, 0, 0, 0, 0, 0, 0, 255, 0, 0, 0, 0,
0, 0, 0, 0, 0, 255, 0, 0, 0, 0, 255, 0, 0, 0, 0, 0,
0, 0, 0, 0, 0, 0, 255, 0, 0, 255, 0, 0, 0, 0, 0, 0,
0, 0, 0, 0, 0, 0, 0, 255, 255, 0, 0, 0, 0, 0, 0, 0,
0, 0, 0, 0, 0, 0, 0, 255, 255, 0, 0, 0, 0, 0, 0, 0,
0, 0, 0, 0, 0, 0, 255, 0, 0, 255, 0, 0, 0, 0, 0, 0,
0, 0, 0, 0, 0, 255, 0, 0, 0, 0, 255, 0, 0, 0, 0, 0,
0, 0, 0, 0, 255, 0, 0, 0, 0, 0, 0, 255, 0, 0, 0, 0,
0, 0, 0, 255, 0, 0, 0, 0, 0, 0, 0, 0, 255, 0, 0, 0,
0, 0, 255, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 255, 0, 0,
0, 255, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 255, 0,
255, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 255,
};
GLuint texture_id;
glGenTextures( 1, &texture_id );
glBindTexture( GL_TEXTURE_2D, texture_id );
glTexImage2D( GL_TEXTURE_2D, 0, GL_RGBA8, 16, 16, 0, GL_LUMINANCE, GL_UNSIGNED_BYTE, buffer );
glTexParameteri( GL_TEXTURE_2D, GL_TEXTURE_MIN_FILTER, GL_LINEAR );
glTexParameteri( GL_TEXTURE_2D, GL_TEXTURE_MAG_FILTER, GL_LINEAR );
The textures are drawn as quads using glBegin/glEnd. The color for each vertex is white with full alpha: {r=255,g=255,b=255,a=255}.
Here's an example scene. The photo and cheese are both loaded from PNG images. The cheese has transparent holes, which show the photo and background behind it. The X pattern I expected to be transparent too:
Why is the quad black instead of transparent, any how can I fix my code to draw what I was expecting?
This question may be similar, but so far I am unable to apply the brief answer to my current problem.
Update: I think I solved it, thanks to the answers below. I changed...
glTexImage2D( GL_TEXTURE_2D, 0, GL_RGBA8, 16, 16, 0, GL_LUMINANCE, GL_UNSIGNED_BYTE, buffer );
to
glTexImage2D( GL_TEXTURE_2D, 0, GL_ALPHA, 16, 16, 0, GL_ALPHA, GL_UNSIGNED_BYTE, buffer );
... which gives the desired result.
#Dietrich Epp's answer was almost correct, only that the format you're looking for is GL_ALPHA:
glTexImage2D(GL_TEXTURE_2D, 0, GL_ALPHA8, 16, 16, 0, GL_ALPHA, GL_UNSIGNED_BYTE, buffer);
Then set the blend function to glBlendFunc(GL_SRC_ALPHA, GL_ONE_MINUS_SRC_ALPHA); or glBlendFunc(GL_ONE, GL_ONE_MINUS_SRC_ALPHA);, depending on if your values are premultiplied. Last but not least set the texture environment to GL_MODULATE
glTexEnvi(GL_TEXTURE_ENV, GL_TEXTURE_ENV_MODE, GL_MODULATE);
Now you can set the color of the 'X' with glColor.
Another approach is not using blending but alpha testing.
Becuse luminance is color, and transparency (alpha) is not color.
If you want transparency, use different format - GL_LUMANANCE_ALPHA or something similar, and store transparency in different channel.
Also, this is already explained in documentation.
GL_LUMINANCE
Each element is a single luminance value. The GL converts it to floating point, then assembles it into an RGBA element by replicating the luminance value three times for red, green, and blue and attaching 1 for alpha. Each component is then multiplied by the signed scale factor GL_c_SCALE, added to the signed bias GL_c_BIAS, and clamped to the range [0,1] (see glPixelTransfer).
--edit--
Any way to keep the 8bit per pixel and achieve the same effect?
I think you could "tell" to OpenGL that initial image is indexed color and then set up proper RGBA palette (where for every element R==G==B==A==index). See glPixelTransfer and glPixelMap for details.
Change GL_RGBA8 (which makes a grayscale image with no alpha channel) to GL_INTENSITY:
glTexImage2D(GL_TEXTURE_2D, 0, GL_INTENSITY, 16, 16, 0,
GL_LUMINANCE, GL_UNSIGNED_BYTE, buffer);
The GL_RGBA8 format, created from GL_LUMINANCE, gives pixels of the form (Y, Y, Y, 1), but GL_INTENSITY with GL_LUMINANCE gives (Y, Y, Y, Y).
You will also want to change your blend mode to assume premultiplied alpha, e.g. change
glBlendFunc(GL_SRC_ALPHA, GL_ONE_MINUS_SRC_ALPHA);
to:
glBlendFunc(GL_ONE, GL_ONE_MINUS_SRC_ALPHA);
Alternative:
You can also use GL_ALPHA, and then use your ordinary blending mode for non-premultiplied alpha:
glTexImage2D(GL_TEXTURE_2D, 0, GL_ALPHA, 16, 16, 0,
GL_ALPHA, GL_UNSIGNED_BYTE, buffer);
Alternative #2:
You can keep using GL_LUMINANCE, and change the blending mode.
glBlendFunc(GL_ONE, GL_ONE_MINUS_SRC_COLOR);
This has the drawback that you can't color the texture without using something like glBlendColor (which is not part of the OpenGL headers that ship with MSVC, so you have to use GLEW or something like that):
glBlendColor(...);
glBlendFunc(GL_CONSTANT_COLOR, GL_ONE_MINUS_SRC_COLOR);
Alternative #3:
Use OpenGL 3, and change your fragment shader to handle single-channel textures in the desired way.
Eight bit images do not use colours, they have a colour palette. Zero represents index 0 of the colour palette for that image, which will contain 256 colours. Eight bit images do not have an alpha channel for transparency. The PNG image you used has an alpha channel. It would be a 32bit image which has 8 bits for red, 8 bits for green, 8 bits for blue (24bits for colour) and 8 bits for alpha. You're mixing the two formats.