Effect of Downsample but not fully divide

Question

Hey I need a 320x240 8Bit gray scale image for some Computer Vision Algorithm (Orb Feature tracking). The Raspicam driver I'm using can provide different Image Sizes. Different Image Sizes are achieved by cropping and not down sampling from the driver. As my environment is not ideal lighted the Image is quite dark and noisy. Now I had the idea to take a 640x480 Image and down sample it to 320x240 by combining always 2x2 pixels to one. Normally I would of course divide by 4 to get the correct result. But what would be the effect of dividing it by two or even one (assuming 99% of the intensity values are not bigger then 64 (256/4)). Wouldn't that simulate the effect of larger CCD cells which could gather more light in less time. The first tests I did showed some pretty good results. Meaning I detected more Features and could follow them better between two frames.

Answer 1

Here, you are not taking proper average of 2x2 blocks(divide by 4). Say, you have two blocks and they have Delta-I difference in intensity. If you divide the intensity of the two blocks by a larger number, the intensity difference will reduce and vice-versa for smaller number.

When you divide the difference(Delta-I) by 2(instead of 4), you are in a way increasing the contrast(intensity difference between background and foreground. As you mentioned that your image is in poor illumination, thereby division by smaller number increases the contrast which is improving tracking. This approach will come under contrast enhancement technique and is a variation of Linear contrast enhancement.