Why do i always get a black image when trying to save the opengl buffer by using freeimage libarary?

Question

Does anybody have experience with using the freeimage library to save the opengl buffer?

Here is my code:

void screenshot(const char *ptr){
    GLint viewPort[4]; 
    glGetIntegerv(GL_VIEWPORT, viewPort);  
    GLubyte *pixels=new GLubyte[3*winWidth*winHeight];
    glPixelStorei(GL_PACK_ALIGNMENT, 1);
    glReadPixels(0, 0, viewPort[2],viewPort[3], GL_RGB, GL_UNSIGNED_BYTE, pixels);  
    FIBITMAP* image = FreeImage_ConvertFromRawBits(pixels,viewPort[2], viewPort[3],3*viewPort[2] 
    ,24,FI_RGBA_RED, FI_RGBA_GREEN_MASK, FI_RGBA_BLUE_MASK,false);
    FreeImage_Save(FIF_BMP, image, ptr, 0);
    FreeImage_Unload(image);
    delete pixels;
}

Why do I always get a black image?

Have you tried doing any debugging at all? Try checking the `pixels` var after you call `glReadPixels`. Make sure you're calling this after you draw your objects and not immediately after `glClear`. — Colonel Thirty Two, Oct 16 '14 at 18:25

score 0 · Answer 1 · answered Oct 17 '14 at 09:07

0

You don't set from which buffer to read from. If you want to take the screenshot after the buffer swap, then you must read from the front buffer. If you want to take the screenshot before the buffer swap, read from the back buffer.

answered Oct 17 '14 at 09:07

datenwolf

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but how can I read the front buffer, simply add a function glreadbuffer(GL_FRONT) before using the screenshot function? – Xiaojun Zhao Oct 17 '14 at 23:13
@XiaojunZhao: Yes, that's exactly how you do it. – datenwolf Oct 18 '14 at 09:00

Why do i always get a black image when trying to save the opengl buffer by using freeimage libarary?

1 Answers1