Drawing the alpha channel correctly in OpenGl - c++

Question

After loading an image, I have the individual bytes for each channel loaded into an array of unsigned characters. It's passed to a function that projects it as a texture onto a quad. Everything seems to work properly other than the alpha channel, which shows up as the background color. I'm using OpenGL to draw the image. Would I benefit by adding a layering mechanism? Also, how can I achieve the transparent effect that I want?
Note: This is the code that I have a feeling needs changed:
void SetUpView()
{
// Set color and depth clear value
glClearDepth(1.f);
glClearColor(1.f, 0.f, 0.f, 0.f);
// Enable Z-buffer read and write
glEnable(GL_DEPTH_TEST);
glEnable(GL_TEXTURE_2D);
glDepthMask(GL_TRUE);
glEnable (GL_BLEND);
glBlendFunc (GL_ONE, GL_ONE_MINUS_SRC_ALPHA);
// Setup a perspective projection
glMatrixMode(GL_PROJECTION);
glLoadIdentity();
gluPerspective(90.f, 1.f, 1.f, 500.f);
};
Also, here's the code to render the quad.
void DrawTexturedRect(RectBounds *Verts, Image *Texture)
{
glBindTexture(GL_TEXTURE_2D, GetTextureID(Texture));
glPixelStorei(GL_UNPACK_ALIGNMENT, 1);
glPixelZoom(1, -1);
glBegin(GL_QUADS);
glTexCoord2f(0.0, 1.0);
glVertex3f(Verts->corners[0].x, Verts->corners[0].y, Verts->corners[0].z);
glTexCoord2f(1.0, 1.0);
glVertex3f(Verts->corners[1].x, Verts->corners[1].y, Verts->corners[1].z);
glTexCoord2f(1.0, 0.0);
glVertex3f(Verts->corners[2].x, Verts->corners[2].y, Verts->corners[2].z);
glTexCoord2f(0.0, 0.0);
glVertex3f(Verts->corners[3].x, Verts->corners[3].y, Verts->corners[3].z);
glEnd();
};
The Image class holds an array of unsigned chars, obtained using OpenIL.
Relevant code:
loaded = ilLoadImage(filename.c_str());
ilConvertImage(IL_RGBA, IL_UNSIGNED_BYTE);
unsigned char *bytes = ilGetData();
//NewImage is an instance of an Image. This is returned and passed to the above function.
NewImage->data = bytes;

Answer 1

Before drawing anything transparent you should call:
glDepthMask(false);
And then afterwards:
glDepthMask(true);
Also, all transparent objects must be drawn after all opaque ones.

Answer 2

glClearColor(1.f, 0.f, 0.f, 0.f);
I'd assume that's at the RGBA default, and you're setting red to 1.0.
How about a better explanation of what you're trying to accomplish?

Answer 3

Try:
glBlendFunc (GL_SRC_ALPHA, GL_ONE_MINUS_SRC_ALPHA);
Edit: I know what that looks like. It looks like you are drawing the blue square (Which has a Z-order position placing it behind the cursor) AFTER the cursor. You have to draw things in the correct, back-to-front Z-Order when alpha blending or you see errors like you are seeing.