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< advanced ~ opengl line not the same size after mirroring |
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| Joined: Sat Feb 21, 2009 6:02 pmPosts: 56 |
hallo people,
at the moment i am testing different opengl actions in openframeworks. for example i draw a line then translate/mirror the scene like this: Code: ofPushMatrix(); ofTranslate(centerX,centerY); for(int j=0;j<2;j++){ glBegin(GL_LINE_STRIP); glVertex2f(0,0); glVertex2f(objectHeight/2,objectHeight/2); glVertex2f(objectHeight/2,objectHeight+underLength-1); glVertex2f(objectHeight/2-1,objectHeight+underLength); glEnd(); ofTranslate(3,0); ofRotateZ(180); ofRotateX(180); ofTranslate(objectHeight,0); } ofPopMatrix(); now i have the problem that the second object dont looks like the first drawn. it seems that all horicontal values are shorten around 1 pixel. i hope that you can understand what i mean. does anyone have an idea what is going wrong here? and maybe someone can tell me if there is a way in opengl to duplicate a once painted object to draw it twice. i hope that it is ok to ask an opengl related question here. //robert |
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| Joined: Sat Feb 21, 2009 6:02 pmPosts: 56 |
hallo again,
here is a pic of what i mean:  the picture schows the painted line 500% increased here is the code to the above picture: Code: ofPushMatrix(); ofTranslate(centerX,centerY); for(int j=0;j<2;j++){ glBegin(GL_LINE_STRIP); glVertex2f(0,0); glVertex2f(10/2,10/2); glVertex2f(10/2,10+10-1); glVertex2f(10/2-1,10+10); glEnd(); ofRotateZ(180); ofRotateX(180); ofTranslate(10,0); } ofPopMatrix(); maybe this describes better what i mean. can someone tell me what i do wrong? |
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 Joined: Fri Jul 04, 2008 9:31 amPosts: 414Location: Vienna, Austria |
i think you might be having rounding issues, since you're passing ints rather than floats. try this:
Code: glBegin(GL_LINE_STRIP); glVertex2f(0,0); glVertex2f(10.0/2,10.0/2); glVertex2f(10.0/2,10.0+10-1); glVertex2f(10.0/2-1,10.0+10); glEnd(); ofRotateZ(180); ofRotateX(180); ofTranslate(10,0);
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| Joined: Sat Feb 21, 2009 6:02 pmPosts: 56 |
hey damian,
thanks for the idea with the rounding. i used float values but this had no effect. than i used an angel of 181° for the rotation. now i get the right result. Code: glBegin(GL_LINE_STRIP); glVertex2f(0,0); glVertex2f(5.0,5.0); glVertex2f(5.0,19.0); glVertex2f(4.0,20.0); glEnd(); ofRotateZ(181); ofRotateX(181); ofTranslate(10,0);  can it be that the the line is 1 pixel behind the position it schould be, when i rotate 180°? |
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| Joined: Sat Feb 21, 2009 6:02 pmPosts: 56 |
i am still confused about the above described.
the confusing is that i can also draw a line backwards instead of mirroring and it also has not the same size and pixel positions as the orginal forward drawn line. this is not a problem if i dont work pixelaccurate but i need each pixel at the right position. maybe opengl is not the right tool for pixelaccurate drawing...please tell me! |
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 Joined: Tue May 27, 2008 10:03 amPosts: 691Location: London, UK |
just out of curiosity, could you try glScalef(-1, 1, 1); instead of two rotations?
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| Joined: Sat Feb 21, 2009 6:02 pmPosts: 56 |
hey memo i tried also your method. thanks for it. but the effect was the same.
maybe it is not possible to draw such a thin 1 pixel line in opengl in such a precise way. i tried 4 different methods with mirroring and without and some other ways...  this is the result. here is the code: Code: void testApp::draw(){ ofPushMatrix(); ofTranslate(1024/2+60,768/2); glBegin(GL_LINES); glVertex2f(30,0); glVertex2f(35.0,5.0); glEnd(); glBegin(GL_LINES); glVertex2f(35.0,5.0); glVertex2f(35.0,19.0); glEnd(); glBegin(GL_LINES); glVertex2f(35.0,19.0); glVertex2f(34.0,20.0); glEnd(); glBegin(GL_LINES); glVertex2f(20,0); glVertex2f(15.0,5.0); glEnd(); glBegin(GL_LINES); glVertex2f(15.0,5.0); glVertex2f(15.0,19.0); glEnd(); glBegin(GL_LINES); glVertex2f(15.0,19.0); glVertex2f(16.0,20.0); glEnd(); ofPopMatrix(); ofPushMatrix(); ofTranslate(1024/2+30,768/2); glBegin(GL_LINE_STRIP); glVertex2f(30,0); glVertex2f(35.0,5.0); glVertex2f(35.0,19.0); glVertex2f(34.0,20.0); glEnd(); glBegin(GL_LINE_STRIP); glVertex2f(20,0); glVertex2f(15.0,5.0); glVertex2f(15.0,19.0); glVertex2f(16.0,20.0); glEnd(); ofPopMatrix(); ofPushMatrix(); ofTranslate(1024/2,768/2); for(int j=0;j<2;j++){ glBegin(GL_LINE_STRIP); glVertex2f(0,0); glVertex2f(5.0,5.0); glVertex2f(5.0,19.0); glVertex2f(4.0,20.0); glEnd(); ofRotateZ(180); ofRotateX(180); ofTranslate(10,0); } ofPopMatrix(); ofPushMatrix(); ofTranslate(1024/2+30,768/2); for(int j=0;j<2;j++){ glBegin(GL_LINE_STRIP); glVertex2f(0,0); glVertex2f(5.0,5.0); glVertex2f(5.0,19.0); glVertex2f(4.0,20.0); glEnd(); glScalef(-1, 1, 1); ofTranslate(10,0); } ofPopMatrix(); } |
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| Joined: Sat Feb 21, 2009 6:02 pmPosts: 56 |
hey hey,
it was a hard way to find this stupid reason but now i have the answer. i found it here: http://www.gamedev.net/community/forums ... _id=556634 a line is not a line, it is a rect. so i positioned it not on 0 but rather on 0.5 and the problem is away. Rectangle: A-----B | | | | C-----D Line made from rectangle: --- |A| | | | | |B| --- maybe this helps someone... |
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