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stella1016

OpenGL texture mapping

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I have one window to display some images. I shoule be able to open any image files, and display it in the middle of the window (the window size if fixed.) I set allowed max image size as (maxTextureWidth, maxTextureHeight). And if opened image size is larger than this size, I will display a scaled version of this image. For reserving the size of ratio, I compute sth of the image and update the texture: { imageWidth = currentImageIpl->width; imageHeight = currentImageIpl->height; int channel = currentImageIpl->nChannels; textureDisplayRatio_x = (float)imageWidth / (float)maxTextureWidth; textureDisplayRatio_y = (float)imageHeight / (float)maxTextureHeight; if(textureDisplayRatio_x > 1.0 || textureDisplayRatio_y > 1.0) { if(textureDisplayRatio_x >= textureDisplayRatio_y) { imageWidth_display = maxTextureWidth; imageHeight_display = imageHeight / textureDisplayRatio_x; } else { imageWidth_display = imageWidth / textureDisplayRatio_y; imageHeight_display = maxTextureHeight; } textureDisplayRatio_x = (float)imageWidth_display / (float)maxTextureWidth; textureDisplayRatio_y = (float)imageHeight_display / (float)maxTextureHeight; } else { imageWidth_display = imageWidth; imageHeight_display = imageHeight; } imageData = currentImageIpl->imageData; if(channel == 3) glTexSubImage2D (GL_TEXTURE_2D, 0, 0, 0, imageWidth, imageHeight, GL_RGB, GL_UNSIGNED_BYTE, imageData); if(channel ==1) glTexSubImage2D (GL_TEXTURE_2D, 0, 0, 0, imageWidth, imageHeight, GL_LUMINANCE, GL_UNSIGNED_BYTE, imageData); update = false; } I did it like this in PaintGL(): (I am using OpenGL with Qt) glLoadIdentity(); {... glTranslatef(GLfloat((maxTextureWidth - imageWidth_display) / 2),GLfloat((maxTextureHeight - imageHeight_display) / 2), 0); glBegin(GL_QUADS); glTexCoord2f(textureDisplayRatio_x, 0.0f); glVertex2f( imageWidth_display, imageHeight_display); glTexCoord2f(0.0f, 0.0f); glVertex2f(0, imageHeight_display); glTexCoord2f(0.0f, textureDisplayRatio_y); glVertex2f(0, 0); glTexCoord2f(textureDisplayRatio_x, textureDisplayRatio_y); glVertex2f( imageWidth_display, 0); glEnd(); ... } The problem is that the image is not displayed in window. What could be the problem? I checked all the computations are right. Thanks in advance.

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Quote:
Original post by Daaark
Did you call glEnable(GL_TEXTURE_2D), and glBindTexture(...)?


Yes, I did glEnable(GL_TEXTURE_2D), but not glBindTexture(...). I only have one texture. So I don't need to bind, right?

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And in initializeGL(), I did this:
glTexImage2D(GL_TEXTURE_2D, 0, GL_RGB, maxTextureWidth, maxTextureHeight, 0, GL_RGBA, GL_UNSIGNED_BYTE, imageData);

Then later I only substitute part of the texture.

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Sorry I didn't mension that, it works well for images which are smaller than my maxTexture size. Only the images bigger than maxTexture size have problem.

OH, I found my problem, the Texture is smaller than my image, I need to new another texture.

:)

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OK. I have a question, how to scale the image to the size which I want it to be? Then I don't have to new another texture, since I have a fixed size texture already. Is there any function supporting this?

what I have are: input size, output size, image data

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Assuming the resizing is just for display purposes, just draw a bigger quad. OpenGL will do the rest for you.

BTW, did you notice that in your PaintGL() function there are only 2 glVertex2f() calls? For a quad you need 4, so no quad would be rendered. ;)

Edit: OK, just overlooked them. They were hidden due to missing line breaks. :(

Edit2: It could also be a matter of winding. Is backface culling disabled?

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Quote:
Original post by Lord_Evil
Assuming the resizing is just for display purposes, just draw a bigger quad. OpenGL will do the rest for you.

BTW, did you notice that in your PaintGL() function there are only 2 glVertex2f() calls? For a quad you need 4, so no quad would be rendered. ;)

Edit: OK, just overlooked them. They were hidden due to missing line breaks. :(


The main problem is that here I only have one texture init. Because I need also display webcam output, in order to make it efficient I used glTexSubImage2D(). Here it looks like I need to create another texture to display...I have to, right?

The best way should be using the present texture, but no way...

For the quad, do you mean draw a quad, and change the viewport or something?

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You actually can use the same texture by just adjusting the texture coordinates, i.e. you use tex coords smaller than 1.

By drawing a quad I just mean what you do already: glBegin(GL_QUADS); ...
Just change the coordinates of the vertices. However, I assume you want a fixed size quad, so playing around with the texture coords would be better.

Just make sure that the texture is fully initialized, i.e. you provide a large enough image with glTexImage(...) or just feed it some all-0 buffer of the same size.

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Quote:
Original post by Lord_Evil
You actually can use the same texture by just adjusting the texture coordinates, i.e. you use tex coords smaller than 1.

By drawing a quad I just mean what you do already: glBegin(GL_QUADS); ...
Just change the coordinates of the vertices. However, I assume you want a fixed size quad, so playing around with the texture coords would be better.

Just make sure that the texture is fully initialized, i.e. you provide a large enough image with glTexImage(...) or just feed it some all-0 buffer of the same size.


Now I understand what you meant. I can change my maxTextureWidth and maxTexutreHeight instead.

What a good idea! I've already created it, and didn't take advantage of it!

Thanks very much.

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