How to glReadPixels properly to write the data into QImage in Linux

Question

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)

Here is after resizing(Weird behaviour)

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?).

Answer 1

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.