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Summary

I want to write the opengl pixels(GL_RGB) by glReadPixels to a QImage.This renders correct, but when i resize the window, it scales weird and distorts my shape(triangle).

What i tried

I tried (QImage)img.scale(width(),height(),Qt::KeepAspectRatio) but it didn't solve the problem.

Played with how i write the pixels buffer from glReadPixels to QImage but No.Didn't work.

Should i read the pixels in three buffers(GLubyte *rpixel,*gpixel,*bpixel) or on one(GLubyte **pixels)?Which one is the easiest because i will resize the array when i will resize my window(so i want dynamic arrays).

Some code

I have uploaded a minimal code recreating the bug-weird behaviour in github.Download and compile using the Qt Creator. https://github.com/rivenblades/GlReadPixelsQT/tree/master

Pictures

Here is how i wanted(it works when not resizing) enter image description here

Here is after resizing(Weird behaviour) enter image description here

As you can see, when resizing, the image gets splitted at right and contunues at left at probably another row.So i am guessing the size of the image is wrong(needs more width?).

Rabbid76
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kostas petsis
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1 Answers1

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By default, the start of each row of an image is assumed to be aligned to 4 bytes. This is because the GL_PACK_ALIGNMENT respectively GL_UNPACK_ALIGNMENT parameter is by default 4, see glPixelStore.
When a framebuffer is read by glReadPixels the GL_PACK_ALIGNMENT parameter is considered.

If you want to read the image in a tightly packed memory, with not alignment at the start of each line, then you've to change the GL_PACK_ALIGNMENT parameter to 1, before reading the color plane of the framebuffer:

glPixelStorei(GL_PACK_ALIGNMENT, 1);

glReadPixels(0,0,unchangable_w, unchangable_h, GL_RED, GL_UNSIGNED_BYTE, tga.rpic);
glReadPixels(0,0,unchangable_w, unchangable_h, GL_GREEN, GL_UNSIGNED_BYTE, tga.gpic);
glReadPixels(0,0,unchangable_w, unchangable_h, GL_BLUE, GL_UNSIGNED_BYTE, tga.bpic);

If that is missed, this cause a shift effect at each line of the image, except if the length of a line of the image in bytes is divisible by 4.

Rabbid76
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  • I added glPixelStorei(GL_PACK_ALIGNMENT,1); and it works correctly when i resize.However, if i resize too much, it cuts the triangle, horizontally if i resize that way, and vertically if the other way.Please, check my code on github to see the problem. [link] (https://github.com/rivenblades/GlReadPixelsQT/blob/master/README.md) – kostas petsis Aug 07 '19 at 16:26
  • Well i figured it out.I needed to call glfwSetWindowSize(GLFWwindow*,m_width,m_height); in the resizeEvent of Qt.Also i needed to pass the GLFWwindow* instance in the widget to do the previous thing. – kostas petsis Aug 07 '19 at 18:37