glBlendFunc cross platform issue

datadream..

glBlendFunc cross platform issue

in Contributed Library Questions • 2 years ago

I am encountering a strange cross-platform issue while using glBlendFunc. I developed this on Windows XP but it will ultimately be running in installation on a MacBook pro. My contact person at the installation site has been testing the software and has noticed a very strange blending problem that doesn't occur on my computer. I have included a comparison screenshot of the issue, with the left side captured on Windows XP and the right side captured on the Mac.

As you can see, the blending works well in Windows, but on the Mac the textures appear to have their transparent areas saturated black. I am using gl.glBlendFunc(GL.GL_SRC_ALPHA, GL.GL_DST_ALPHA); to accomplish this type of blend. I am using GLGraphics as the renderer. The texture is being rendered as a Texture object like so:

// draw indicator
gl.glPushMatrix();
gl.glTranslatef(x, y, 0);
gl.glBlendFunc(GL.GL_SRC_ALPHA, GL.GL_DST_ALPHA);
gl.glColor4f(r, g, b, a);
texture.bind();
texture.enable();
gl.glBegin(GL.GL_QUADS);
gl.glTexCoord2f(0, 0);
gl.glVertex2f(-diameter/2, -diameter/2);
gl.glTexCoord2f(1, 0);
gl.glVertex2f(diameter/2, -diameter/2);
gl.glTexCoord2f(1, 1);
gl.glVertex2f(diameter/2, diameter/2);
gl.glTexCoord2f(0, 1);
gl.glVertex2f(-diameter/2, diameter/2);
gl.glEnd();
texture.disable();
gl.glPopMatrix();

My machine is running Windows XP 64-bit with a GeForce 9800GT. The mac is running Snow leopard 10.6.8, has an I ntel core i7 2.2ghz, and AMD Radeon HD 6750m and Intel HD Graphics 3000. I unfortunately don't have a mac of my own to test different blend options on to see what will work, so I am relying on hopefully somebody having encountered a similar cross-platform issue before.

Replies(8)

kooogy

Re: glBlendFunc cross platform issue

2 years ago

can you post your texture? and perhaps a close-up of the affected areas - it's hard to make out on that scale.

i may have seen something similar before - i was using a texture with a radial fade. but i'd only faded the colour. and when i combined a bunch of these i got a dark line where they overlapped. this went away when i faded the alpha channel as well as the colour. (texture was generated in the code, rather than just a png)

http://www.flickr.com/photos/31962137@N00/4761204981/in/set-72157594524240895 (see attached note)
http://www.flickr.com/photos/31962137@N00/1103167546/in/set-72157594524240895 (also visible here)

kooogy

Re: Re: glBlendFunc cross platform issue

2 years ago

public static final int TSIZE = 128;
void createTex() {
img = createGraphics(TSIZE, TSIZE, P3D);
img.beginDraw();
float c;
for (int x = 0 ; x < TSIZE ; x++) {
int dx = ((TSIZE / 2) - x) * ((TSIZE / 2) - x);
for (int y = 0 ; y < TSIZE ; y++) {
int dy = ((TSIZE / 2) - y) * ((TSIZE / 2) - y);
float d = sqrt(dx + dy) / 64; // d = 0 - 1
if (d < 1) {
c = 255 * (1.0 - (d * d * d));
img.stroke(c, c, c, c);
img.point(x, y);
}
}
}
img.endDraw();
//tex.save("tex.png");
}

this is the code i ended up using to generate the texture. note the use of r, g, b and a in the img.stroke()

datadream..

Re: glBlendFunc cross platform issue

2 years ago

Thanks for the response Koogy. The first link is unfortunately set to private, so I'm not sure what it displays. The second link appears to be a successful blending, and I don't see any artifacts like the ones I'm encountering.

Unfortunately, I can not increase the size because the screenshot taken on a mac is from a build installed on a computer 300 miles away, and for me to see how it looks on that I have to make changes, build it, send it off, and hope my contact person is around to test it out on site. The textures are always being drawn very small, as they are indicators of people's locations on the map. They start out around 30px and tween down to 10px over a second.

This is the image I'm using:

You can't see it, because it's a soft spherical gradient with white on the inside and transparency on the outside. I load it in as an opengl Texture object, using the following code:

try {
indicatorTexture = buildTexture(TextureIO.newTextureData(ImageIO.read(new File("data/"+props.getProperty("indicatorTexture"))), true));
} catch (IOException e) {
e.printStackTrace();
}

Where buildTexture does this:

private Texture buildTexture(TextureData td){
Texture tex = TextureIO.newTexture(GL.GL_TEXTURE_2D);
tex.setTexParameteri(GL.GL_TEXTURE_MAG_FILTER, GL.GL_LINEAR);
tex.setTexParameteri(GL.GL_TEXTURE_MIN_FILTER, GL.GL_LINEAR);
tex.setTexParameteri(GL.GL_TEXTURE_WRAP_S, GL.GL_CLAMP_TO_EDGE);
tex.setTexParameteri(GL.GL_TEXTURE_WRAP_T, GL.GL_CLAMP_TO_EDGE);
tex.bind();
tex.updateImage(td);
tex.disable();
return tex;
}

This is some recycled Texture loading code that I've used on a couple projects in the past, both running on Mac OS X, so I would be surprised if that was the culprit.

The only thing new that I have done that I have not previously utilized on Mac OS X is the gl.glBlendFunc(GL.GL_SRC_ALPHA, GL.GL_DST_ALPHA); blend mode.

kooogy

Re: glBlendFunc cross platform issue

2 years ago

it's always a pain when you can't debug it yourself. and looking closer i can see the problem, a black(er) square behind the glow. not the thing i was seeing, but maybe related (and i've made that first image public now)

your dot looks ok but i haven't been able to check the alpha values. background colour is white though. mine are usually black. not sure if that matters. if you run the code i posted (and uncomment the save line, change it to img.save) it'll create you the exact image i use. can you try that?

also, in my circles example i'm using
gl.glBlendFunc(GL.GL_SRC_ALPHA, GL.GL_ONE);
but am not sure of the difference that'd cause. there was a handy chart.

this isn't it: https://forum.processing.org/topic/additive-blending (but confirms the above)

ah, this is: http://zanir.wz.cz/programming/opengl/img/011_blending.jpg

kooogy

Re: Re: glBlendFunc cross platform issue

2 years ago

actually, that chart does show the difference - look at the GL_SRC_ALPHA row (which should be GL_DST_ALPHA as it's the desitination), see how the background is darker than the GL_ONE row. isn't this what you're seeing, that the backgound is darker under your texture?

datadream..

Re: glBlendFunc cross platform issue

2 years ago

Well, I changed my blend mode, and it looks exactly the same on my Windows XP machine, which is good. I will make a build and send it to my contact who should hopefully be setting things up right now for this installation. Really hope this works out.

datadream..

Re: glBlendFunc cross platform issue

2 years ago

Just got word from my contact that the GL_SRC_ALPHA, GL_ONE blending works as it should. :)

Thanks for the help. It came just in time for the conference opening.

kooogy

Re: Re: glBlendFunc cross platform issue

2 years ago

cool

can't really explain why it looked ok on windows.

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