3

I am using GLUT for my windowing system to output some images onto the screen. I also want to output the images to files. This has been a challenge. In some window resolution (such as 256x256, 1024x1024, 1920x1200), glReadPixels produce exact same image files as the screen. However when I resize the window (ex/ 655x652, 529x566 and do the screen capturing, images are zagged and seems like pixels have shifted linearly.

Any help will be appreciated!

EDIT - I am adding some code leading up to the glReadPixels call. 

glGetIntegerv(GL_VIEWPORT, dimensions);
width = dimensions[2];
height = dimensions[3];
screencapture = (unsigned char*)malloc(width*height*sizeof(unsigned char));
glReadPixels(0,0,width, height, GL_GREEN, GL_UNSIGNED_BYTE, screencapture);

EDIT2 - I forgot to mention resizing the window with the mouse some times do produce good images. I guess it happens when I get the resolution just right.

genpfault
  • 51,148
  • 11
  • 85
  • 139
Adam Lee
  • 2,983
  • 7
  • 35
  • 47
  • 3
    Assuming that you've not been playing with `glPixelStore` (e.g. `GL_PACK_ALIGNMENT`) this should work just fine (works for me, anyway). The description sounds like you're assuming a wrong width when saving the pixel data. Maybe some code might help (it's hard to tell what goes wrong, not knowing what exactly you do). – Damon Jul 06 '11 at 16:26
  • Hi Damon, thanks for help. I just added some code. And yes you are right I am not playing with GL_PACK_ALIGNMENT. – Adam Lee Jul 06 '11 at 17:04
  • 1
    There we go. You are reading only the green channel, i.e. you are reading __bytes__. GL_PACK_ALIGNMENT has an [initial value of 4](http://www.opengl.org/sdk/docs/man3/xhtml/glGet.xml) (a reasonable default because most people read 32 bit values). Which means that on every non-multiple-of-4 width, you have extra bytes at the end of the line (if you don't account for those, you get the following lines offset). Is it accidential or deliberate that you only read one channel? – Damon Jul 06 '11 at 17:10
  • Reading the green channel is intended. I am producing monochrome images for now. So only the width matters? Height can be non-multiple of 4? Thanks Damon for your help! – Adam Lee Jul 06 '11 at 17:12

1 Answers1

3

The solution to your problem would be to either account for the extra bytes at the end of each line that is not a multiple of 4 wide (skipping over them), or to change GL_PACK_ALIGNMENT to 1 (using glPixelStore).

(see my above comment)

Damon
  • 67,688
  • 20
  • 135
  • 185