OpenGL transparent texture messing with GL_Lines

Genghiz Cohen

Right, messing with GL. When I use a .tga texture with no transparency then everything is fine.
All my debug GL_Lines draw fine.

But when I tried loading a .tga with an alpha channel all my LG_Lines seem to be drawn in black.


I am setting the colour of the lines with glColor4f();

Here is the init function for the graphics:

void InitGraphics()
{
    hDC = GetDC(hWnd);

    SetupPixelFormat();

    hRC = wglCreateContext(hDC);
    wglMakeCurrent(hDC, hRC);

	glEnable(GL_TEXTURE_2D);
	glShadeModel(GL_SMOOTH);
	glClearColor(0, 0, 0, 0.5);
	glClearDepth(1.0);
	glEnable(GL_DEPTH_TEST);
	glDepthFunc(GL_LEQUAL);
	glHint(GL_PERSPECTIVE_CORRECTION_HINT, GL_NICEST);
    
}

And the draw calls, the gl_Blend doesn't seem to do anything. If I remove them (and they usually are removed) nothing changes.

void DrawGraphics()
{
    glClear(GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT);

    // Set location in front of camera
    glLoadIdentity();
    glTranslated(0, 0, -10);

    // Draw shapes
	glEnable(GL_BLEND);
	glBlendFunc(GL_SRC_ALPHA,GL_ONE);
	ShapeManager::Instance()->DrawShapes();
	glDisable(GL_BLEND);
	#ifdef DEBUG
		DrawDebug();
	#endif

    // Show the new scene
    SwapBuffers(hDC);
}

DrawShapes is just a loop calling glVertex3f on a load of gl_Quads.

Any ideas, most of this code is stolen, blatantly, from NeHe (Praised be his name).

fasty

It's ages since I've looked at fixed function OpenGL so I'm guessing there, but what alpha do you set for the GL_Lines?

Genghiz Cohen

fasty wrote: »

It's ages since I've looked at fixed function OpenGL so I'm guessing there, but what alpha do you set for the GL_Lines?

1

Also when I said GL_Quads, I meant GL_Polygon.

Genghiz Cohen

Also,
If the bottom right corner of a texture is a colour, sometimes the debug grid in the background gets drawn with what appears to be that colour mixed with the glColor4f(0.2,0.2,0.2,0.5); I set the line to.

Plus, when using multiple shapes with my default texture (.png here because boards doesn't like .tga)


If one passes over the other, the white sections show through.

Genghiz Cohen

And NOW I notice that a texture with transpatency over one without blends them together.

fasty

Hmm, your blend function setup doesn't seem right to me now I'm looking at it. It should be GL_SRC_ALPHA, GL_ONE_MINUS_SRC_ALPHA.

This means that if a source pixel's alpha is 0, the colour will be source.rgb * 0. This will be added to destination.rgb * (1 - 0), which is whatever you set the clear colour to in this case.

glBlendFunc(GL_SRC_ALPHA,GL_ONE);

It doesn't matter in in this case though, because you're essentially using the alpha of your textures as a mask.

Genghiz Cohen

fasty wrote: »

It doesn't matter in in this case though, because you're essentially using the alpha of your textures as a mask.

Almost.
Without the debug lines, everything draws correctly now. So that's a big step, thanks!

srsly78

Just force alpha to 1 if not using an alpha texture. Or disable blending altogether if you aren't using it.

Those lines are for an overlay right? If it's an overlay you don't want to blend at all, just use the "painter's algorithm" and draw it last. If it's a screenspace overlay use a special 2d shader with normalised device coordinates, if the lines exist in 3d space then just draw them last and disable depth test.

Genghiz Cohen

srsly78 wrote: »

if the lines exist in 3d space then just draw them last and disable depth test.

Will try this tomorrow.
Is it wrong for a man to hope?

Genghiz Cohen

Bah, no joy.

srsly78

Get rid of all transparency, no blending at all. What does it render as then?

Does your texture have an alpha channel? Is it RGBA?

When I get stuck I usually go back to basics, and add complexity until it breaks. Try using per-vertex colored quads instead of textures, should be easier to debug.

Genghiz Cohen

srsly78 wrote: »

Get rid of all transparency, no blending at all. What does it render as then?

Does your texture have an alpha channel? Is it RGBA?

When I get stuck I usually go back to basics, and add complexity until it breaks. Try using per-vertex colored quads instead of textures, should be easier to debug.

If I take out all texturing and blending code and let a texture load, it's the same result.
If the texture has Alpha, all lines are drawn black.
If not, lines are drawn correctly and the GL_Polys come up as filled white (as I set the GL_Color to white)

Looking into my (NeHe's) tex loading code, I commented out these lines and the issue went away

// Typical Texture Generation Using Data From The TGA ( CHANGE )
		glGenTextures(1, &texture->texID);				// Create The Texture ( CHANGE )
		glBindTexture(GL_TEXTURE_2D, texture->texID);
		glTexImage2D(GL_TEXTURE_2D, 0, texture->bpp / 8, texture->width, texture->height, 0, texture->type, GL_UNSIGNED_BYTE, texture->imageData);
		glTexParameteri(GL_TEXTURE_2D,GL_TEXTURE_MIN_FILTER,GL_LINEAR);
		glTexParameteri(GL_TEXTURE_2D,GL_TEXTURE_MAG_FILTER,GL_LINEAR);

fasty

Can you post the line drawing code, including any state you might set? The texture loading code looks fine, proviced the bpp is 32 for RGBA TGAs and 24 for RGB.

Genghiz Cohen

The for loop is temporary until I get around to telling it to only draw on visible space where X % 10 == 0 and Y %10 == 0

void DrawDebug()
{
	glPopMatrix();
	glBegin(GL_LINES);

	glColor4f(0.5,0.5,0.5,1);

	for(int i = 0; i < 2000; i++)
	{
		glVertex3f(-1000 + 10 * i, 1000, 1);
		glVertex3f(-1000 + 10 * i, -1000, 1); 

		glVertex3f(1000, -1000 + 10 * i, 1);
		glVertex3f(-1000, -1000 + 10 * i, 1);
	}
	
	glColor4f(1,0,0,1);
	glVertex3f(0,0,1);
	glColor4f(1,0,0,1);
	glVertex3f(1,0,1);
	
	glColor4f(0,1,0,1);
	glVertex3f(0,0,1);
	glColor4f(0,1,0,1);
	glVertex3f(0,1,1);

	glColor4f(0,0,1,1);
	glVertex3f(0,0,1);
	glColor4f(0,0,1,1);
	glVertex3f(0,0,2);
	glEnd();
	glPopMatrix();

	
	SManager->DrawDebug(); // Tell the ShapeManager to draw its debug lines
	
}

I'm sitting here cringing, waiting for you go to "Um, yeah.. It's that super obvious error on line derp..."

Genghiz Cohen

If I glClearColor(1, 0, 0, 0);
Then the GL_Lines and polys stay black.

fasty

Haha, no major derps that I can see so far but I'll have a think about it.

It's a strange one. For my framework, I have a similar debug drawing interface that lets me draw lines and shapes in 2D or 3D on top of the final scene. All I do is disable depth testing to get stuff to appear on top of everything else, or just disable depth writes to make the debug lines work properly with 3D space.

I've never had any weird alpha issues like this at all!

Genghiz Cohen

I could put up the source code if that would be useful (keeping in mind it's poorly commented and heavily experimental)

fasty

Yeah, I'll take a look at the source if you want to post it. Don't worry about comments or experiments, it's impossible to figure things out without hacking out messy code first!

Genghiz Cohen

fasty wrote: »

Yeah, I'll take a look at the source if you want to post it. Don't worry about comments or experiments, it's impossible to figure things out without hacking out messy code first!

http://www.filedropper.com/glfirst

There's a super fancy algo for generating randomly shaped pixely asteroids in there, don't be stealing it :P

srsly78

Umm that's ancient OpenGL I haven't seen glBegin/glEnd in years! These days I mostly use OpenGL ES on mobiles or DirectX on desktop. Am in work so can't look too closely right now tho

Your glClearColor() however... Are you using RGBA or ARGB? You realise you set alpha to zero right? That's why everything went black.

Genghiz Cohen

srsly78 wrote: »

Umm that's ancient OpenGL I haven't seen glBegin/glEnd in years! These days I mostly use OpenGL ES on mobiles or DirectX on desktop. Am in work so can't look too closely right now tho

Your glClearColor() however... Are you using RGBA or ARGB? You realise you set alpha to zero right? That's why everything went black.

It's the GL we learned, I mean to change it to arrays at some point.

Changing the clear colour A value didn't seem to do anything last time I tired, and even if it did, how would that effect the lines getting drawn on top of it?

fasty

I'll hit it with the debug stick later on! A quick look at the code doesn't show anything obvious!

fasty

The problem was the state machine aspect of OpenGL. GL_TEXTURE_2D was enabled when drawing your debug lines, but you didn't specify texture coordinates or anything.

Since you'll only be drawing debug lines in debug mode, wrap that drawing with

void DrawDebug()
{
	glDisable(GL_TEXTURE_2D);
	// your debug drawing code
	glEnable(GL_TEXTURE_2D);
}

Try to minimise state changes in actual rendering code if you can! Oh, and you can move all that other stuff to your init function. Just make sure you give the lines and alpha of 1 and you won't need to switch blending on and off.

BTW, If you want to move on to newer versions of OpenGL and use cool stuff like GLSL, VBOs and FBOs, here are some great sites:

http://duriansoftware.com/joe/An-intro-to-modern-OpenGL.-Table-of-Contents.html
http://www.arcsynthesis.org/gltut/

You lose things like the matrix stack (a good thing IMO), so you'll need a maths library. GLM is worth a look, as is the Configurable Maths Library.

And there's this article on using GLEW to create a 3.1 context

http://www.opengl.org/wiki/Tutorial:_OpenGL_3.1_The_First_Triangle_(C%2B%2B/Win)

or a minimal version of my own device class here:

http://pastebin.com/mBjR3YLi

and Swiftless' OpenGL 4 tutorials

http://www.swiftless.com/opengltuts/opengl4tuts.html

But don't worry about using fixed function OpenGL for now, it does the job if you just want to play with the gameplay stuff.

Genghiz Cohen

fasty wrote: »

The problem was the state machine aspect of OpenGL. GL_TEXTURE_2D was enabled when drawing your debug lines, but you didn't specify texture coordinates or anything.

Since you'll only be drawing debug lines in debug mode, wrap that drawing with

void DrawDebug()
{
	glDisable(GL_TEXTURE_2D);
	// your debug drawing code
	glEnable(GL_TEXTURE_2D);
}

Try to minimise state changes in actual rendering code if you can! Oh, and you can move all that other stuff to your init function. Just make sure you give the lines and alpha of 1 and you won't need to switch blending on and off.

BTW, If you want to move on to newer versions of OpenGL and use cool stuff like GLSL, VBOs and FBOs, here are some great sites:

http://duriansoftware.com/joe/An-intro-to-modern-OpenGL.-Table-of-Contents.html
http://www.arcsynthesis.org/gltut/

You lose things like the matrix stack (a good thing IMO), so you'll need a maths library. GLM is worth a look, as is the Configurable Maths Library.

And there's this article on using GLEW to create a 3.1 context

http://www.opengl.org/wiki/Tutorial:_OpenGL_3.1_The_First_Triangle_(C%2B%2B/Win)

or a minimal version of my own device class here:

http://pastebin.com/mBjR3YLi

and Swiftless' OpenGL 4 tutorials

http://www.swiftless.com/opengltuts/opengl4tuts.html

But don't worry about using fixed function OpenGL for now, it does the job if you just want to play with the gameplay stuff.

In years to come, when developers run into a GL problem, they will whisper a short prayer to fasty.
You legend.