How do I grab an image from my EAGLLayer ?

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温柔的废话
温柔的废话 2021-01-06 21:45

I\'m looking for way to grab the content of my opengl (as UIImage) and then save it into a file. I\'m now giving glReadPixels a try though I\'m not sure I\'m doing the right

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  • 2021-01-06 22:06

    Look up "Technical Q&A QA1704 OpenGL ES View Snapshot", where Apple gives a worked example, including Nils' "better way" to unflip the image. It gets you to your UIImage, and from there follow Nils for a means of saving as a file.

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  • 2021-01-06 22:14

    All OpenGL|ES complient GL-implementations have to support GL_RGBA as a parameter to glReadPixels.

    If your OpenGL|Es supports the

    GL_IMPLEMENTATION_COLOR_READ_FORMAT_OES 
    

    extension you can also query the native format. glReadPixels will understand this format as a parameter to glReadPixels and allows you to directly get the pixel-data in native format. This is usually a bit faster than GL_RGBA.

    Take a look at the header-file or query the extension-support via glGetString.

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  • 2021-01-06 22:16

    Get the data out of the OpenGL ES view:

    -(UIImage *) snapshot {
        NSInteger myDataLength = backingWidth * backingHeight * 4;
        // allocate array and read pixels into it.
        GLubyte *buffer = (GLubyte *) malloc(myDataLength);
        glReadPixels(0, 0, backingWidth, backingHeight, GL_RGBA, GL_UNSIGNED_BYTE, buffer);
        // gl renders "upside down" so swap top to bottom into new array.
        // there's gotta be a better way, but this works.
        GLubyte *buffer2 = (GLubyte *) malloc(myDataLength);
        for(int y = 0; y <backingHeight; y++)
        {
            for(int x = 0; x <backingWidth * 4; x++)
            {
                buffer2[((backingHeight - 1) - y) * backingWidth * 4 + x] = buffer[y * 4 * backingWidth + x];
            }
        }
        free(buffer);
        // make data provider with data.
        CGDataProviderRef provider = CGDataProviderCreateWithData(NULL, buffer2, myDataLength, NULL);
        // prep the ingredients
        int bitsPerComponent = 8;
        int bitsPerPixel = 32;
        int bytesPerRow = 4 * backingWidth;
        CGColorSpaceRef colorSpaceRef = CGColorSpaceCreateDeviceRGB();
        CGBitmapInfo bitmapInfo = kCGBitmapByteOrderDefault;
        CGColorRenderingIntent renderingIntent = kCGRenderingIntentDefault;
        // make the cgimage
        CGImageRef imageRef = CGImageCreate(backingWidth, backingHeight, bitsPerComponent, bitsPerPixel, bytesPerRow, colorSpaceRef, bitmapInfo, provider, NULL, NO, renderingIntent);
        // then make the uiimage from that
        UIImage *myImage = [UIImage imageWithCGImage:imageRef];
        return myImage;
    }
    

    Then to save the image:

    -(void) saveImage:(UIImage *)image {}
        // Create paths to output images
        NSString  *pngPath = [NSHomeDirectory() stringByAppendingPathComponent:@"Documents/Test.png"];
        NSString  *jpgPath = [NSHomeDirectory() stringByAppendingPathComponent:@"Documents/Test.jpg"];
    
        // Write a UIImage to JPEG with minimum compression (best quality)
        // The value 'image' must be a UIImage object
        // The value '1.0' represents image compression quality as value from 0.0 to 1.0
        [UIImageJPEGRepresentation(image, 1.0) writeToFile:jpgPath atomically:YES];
    
        // Write image to PNG
        [UIImagePNGRepresentation(image) writeToFile:pngPath atomically:YES];
    }
    
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