Lesson video

Hello, my name is Mr. Hogan.

I'm so excited to be learning with you today.

We are going to have such a great time learning all about data representation of images and sounds.

I'll be supporting you with our learning during these lessons.

I'm so pleased that you have decided to complete your learning with me today.

We are going to do brilliantly.

Welcome to today's lesson from the unit, Data Representation: Images and Sound.

This lesson is called File Size of Digital Images.

I'm really looking forward to this lesson.

And by the end of it, you'll be able to describe how image file sizes are calculated and how changes to image quality and colour accuracy adjust this.

Let's move on.

I'm really looking forward to it.

So there are three key words in today's lesson.

So resolution.

This is dimensions of an image in pixels or the number of pixels per inch, sometimes known as PPI, which affects image quality.

Colour depth.

This is the number of bits used to represent a colour, which is also known as bit depth.

And then our final keyword is file size.

So this is the amount of data a file contains.

Remember, you can come back to this slide at any time if you forget what these keywords mean by just going and rewinding the video.

So the lesson splits into three parts.

The first part is calculating the file size of an image.

Then we move on to the effects of using different image resolutions.

And then finally, the effects of using different colour depths in an image.

So let's start with calculating the file size of an image.

A digital image is composed of individual elements arranged in a rectangular grid.

The elements of a digital image are called pixels.

A number of pixels in a digital image is the image resolution.

To work out the resolution in pixels, we take the width in pixels and multiply it by the height in pixels.

So where the width is 720 pixels and the height is 720 pixels, like in this image, we calculate the resolution by multiplying the width and height together to give the number of pixels.

In this case, 518,400 pixels in this picture's res resolution.

Let's have a quick check.

See if you can calculate the resolution of this image.

Is it A, 112 pixels, B, 22 pixels, or C, 100 pixels? You may pause the video.

Maybe you want to do some counting of pixels.

So you can pause the video or you can rewind it to see how you work out the resolution and then come back to this check.

Let's have a look at the answer.

It is A, 112 pixels.

That's because the width is 8 pixels, the height is 14 pixels.

So to work out the resolution, we have 8 multiplied by 14, which equals 112 pixels.

To calculate the number of bits in an image that uses 2 bits per pixel, we do the following.

So there are 10 by 10 pixels, so therefore we multiply 'em together, which gives 100 pixels or picture elements.

And 2 bits are used for the colour of each pixel.

You can now see in the image.

Total number of binary digits for the image is worked out by taking the hundred pixels, multiplying it by 2 bits per pixel.

So this gives us 200 bits.

So that's how to calculate the number of bits in an image that uses 2 bits per pixel.

For every pixel, a sequence of binary digits is used to represent its colour.

The number of binary digits used to represent each pixel colour is the colour depth.

So the resolution of this image is worked out by multiplying the width and height together.

So that was 518,400 pixels.

And the colour depth is 1 bit because it's two possible colours, there's only black or white represented by 1 and 0.

It may look a bit grey, but actually if you zoom in, you'll be able to see that it's just formed of black and white pixels.

If we add more colours to the image, so like the grey represented by 1-1 and the blue represented by 1-0, the colour depth is 2 bits.

Four possible colours.

If we add eight colours represented by 3 bits, the colour depth is now 3 bits.

This image actually has 24 bits for its colour depth, and that's a typical value of a image's colour depth.

Something like 24 or sometimes 32 binary digits per pixel.

Let's have a check.

What does the colour depth of an image describe? Is it A, the height of the image in pixels, B, the number of different colours it can show, or C, the number of different colours such as red, green, and blue that are combined in each pixel? You can pause the video at any time or rewind it to look at previous slides.

Let's have a look at the answer.

It is B, the number of different colours it can show.

Moving on.

To calculate the file size of an image, we do the following.

So we take the width and pixels, multiply it by the height and pixels, and then multiply it by the colour depth.

So let's have a look at this image.

So we've got 720 pixels, height and width, and we multiply it by 24 bits 'cause it's got 24 bit colour depth like we looked at before.

So that gives us 12,441,600 bits.

Sam is asking, "12,441,600 bits.

That's a huge number! Is there a way to convert it into other units to make it easy to understand?" Yes, there are, Sam.

There are more common units of digital storage.

So we use a byte.

And in a byte, there are eight bits.

So if we do 12,441,600 divided by 8, it becomes 1,555,200 bytes.

1,000 bytes in a kilobytes.

So we can do, 1,555,200 bikes divided by 1,000 gives us 1,555 kilobytes.

And 1,000 kilobytes are in a megabyte.

So we can do 1,555 kilobytes divided by 1,000, so that gives us 1.

55 megabytes.

Let's have a practise.

One, "Sam has made a logo for a game.

The logo is a bitmap image of 3,000 by 3,000 pixels.

What is the logo's resolution?" Two, "Sam has also made an icon for the maze solving game she has created.

It is 32 pixels wide and 32 pixels high.

How many pixels does the image have in total?" And maybe you can pause the video at any time or rewind it.

Three, "In Sam's game, there is a character that is 32 pixels tall by 16 pixels wide.

The games console can represent 64 colours, which is represented by 6 bits per pixel.

What is the file size of the character in bites?" Remember, pause the video anytime or rewind it.

Let's have a look at the answers.

We've got one, "Sam has made a logo for a game.

The logo is a bitmap image of 3,000 by 3,000 pixels.

What is the logo's resolution?" So we take the height and width and multiply them together.

So 3,000 pixels multiply by 3,000 pixels equals 9 million pixels.

Two, "Sam has also made an icon for the maze solving game she has created.

It is 32 pixels wide and 32 pixels high.

How many pixels does the image have in total?" So, similarly to the last question, we take the height and width, multiply 'em together.

So in this case, we get 32 pixels multiply by 32 pixels, and that gives us 1,024 pixels.

"In Sam's game, there is a character that is 32 pixels tall by 16 pixels wide.

The game's console can represent 64 colours, which is represented by six bits per pixel.

What is the file size of the character bytes?" Okay, so we can work out the resolution.

So 32 by 16 pixels gives us 512 pixels.

And we can work out now the total bits because we know that 512 pixels multiplied by the colour depth, which in this case is 6 bits.

So that gives us 3,072 bits.

Now we convert the bits into bytes.

So 3,072 bits divided by 8 gives us 384 bytes.

Well done if you've got this correct.

Really good.

Let's move on to the second part of the lesson.

So, we're gonna look at the effects of using different image resolutions.

So we've calculated the file size of an image.

Now we're looking at the effects on using different resolutions.

When choosing a resolution size for an image, there is a balance between quality and efficiency.

Quality refers to the clarity and the detail of an image and high quality image are sharp and have really fine details in them.

Efficiency means doing something well without wasting resources like storage space, internet speed, or computing power.

For example, efficient images are changed and improved to make sure they look good without using too much space or slowing things down on your computer.

Why resolution matters? Well, in gaming and graphic design, resolution affects how realistic the game looks.

In photography, higher resolution images makes the photographs more detailed.

And also in streaming or when using images on the web, choosing the right resolution balances this quality and efficiency.

Let's have a quick check.

"What is the main benefit of using a high resolution for an image?" Is it A, the image will use less storage space? Is it B, the image will appear clearer and more detailed? Or is it C, the image will load faster on our website? Remember, you can pause the video at any time.

Let's have a look at the answer.

It is B, the image will appear clearer and more detailed.

Sam is asking, "For my schoolwork, I don't look at the resolution of images that I use." Sofia is commenting, "I was creating a presentation and when I made the image larger, it became blurry so I had to find an image with a higher resolution." That's a good comment by Sofia.

I've come across images where I've enlarged them and they've become blurry.

So I've had to actually go and find images and look at their resolution.

This image is 2,560 pixels by 2,027 pixels.

So in total, this resolution is 5,189,120 pixels.

Let's have a look at this next one.

So this resolution has gone down slightly.

So it's 1,280 pixels by 1,014 pixels, which gives us a resolution of 1,297,920 pixels.

Okay.

Now let's have a look at this next image.

So this is slightly decreased in its resolution.

So this is 970 pixels by 768 pixels, which gives 744,960 pixels.

Okay, so the resolution has reduced.

This one's reduced even more.

So the resolution of this is 453,443 pixels.

This one's reduced even more.

I wonder if you can start telling that the quality's gone down.

So this one's 290,880 pixels.

This one's gone down.

So this is only 49,500 pixels.

This one's even less.

Now this one's 7,900 pixels.

I wonder if you can tell the difference now between this one and the original one.

So let's put 'em side by side and hopefully you can see that the quality is different.

Although we sort of went through the slide and decrease the resolution or the quality at each slide, if you put our best resolution image with 5,189,120 pixels on the right-hand side next to our image with our least resolution, which is 7,900 pixels on the left-hand side, hopefully you can tell the difference.

So let's have a look at visual quality.

Higher resolution improves detail, especially on larger screens.

Low resolution images may look pixelated or lack detail when zoomed in or displayed on high definition monitors.

When looking at file size, images with higher resolutions have larger file sizes because they store more pixels.

Lower resolution images take up less storage but are lower quality.

Let's have a look at examples.

Higher resolution images are ideal for printing, professional photography, and really large displays.

Low resolution images are suitable for web use, faster loading times, and devices with smaller screens.

Let's have a check.

"Sofia wants her website to load quickly.

Which image resolution should she choose to ensure fast loading times while maintaining good quality?" Is it A, high resolution, which often leads to slower loading times, B, low resolution which may load quickly, but could compromise image quality, Or C, optimise resolution, balancing quality and loading speed.

Remember, you can pause to have a think about the answer or rewind the video.

The answer is C, yeah, optimise resolution, balancing that quality and loading speed.

Well done for getting this far.

It's time for a practise, but you're doing really, really well.

Let's see if we can answer these questions.

One, "Sofia is sending a photo attached to an email to Sam.

Why might Sofia choose to send a low resolution version of the photo instead of the original high resolution version?" Two, "Sam is printing a large photo to hang on her wall.

Should Sam choose a high resolution or low resolution image? Explain your answer." Now, you can pause the video at any time or rewind to look at previous slides.

Three, "Sofia has been asked to design an image to promote a healthy lifestyle on a digital billboard to be displayed in a city centre.

What factors would you consider when choosing the resolution of the images for the billboard? Explain your reasoning." Remember, you can pause the video at any time.

Let's have a look at the answer for number one.

"So Sofia is sending a photo attached to an email to Sam.

Why might Sofia choose to send a low resolution version of the photo instead of the original high version? So sending a low resolution photo through an email is better because it'll have a smaller file size.

This means it will send faster and be less likely to cause problems with email size limits.

While the quality might not be as good as the original, it will still be fine for viewing on a phone or a computer screen.

Hopefully you've got something very similar to this answer.

Well done.

Two, "Sam is printing a large photo to hang on her wall.

Should Sam choose a high resolution or low resolution image? Explain your answer." So Sam should choose a high resolution image for printing a photo.

High resolution images have more pixels, which means the image will be printed with more detail and clarity.

A low resolution image might look blurry or pixelated when printed in a large size.

Again, hopefully you've got the main points of these answers.

Three, "What factors should you consider when choosing the resolution of the images for the billboard?" I would choose a high resolution.

Billboards are very large, so the images need to have a lot of pixels to look clear and shut from a distance.

Billboards are viewed from a distance, so image quality is prioritised over file size.

However, I would also need to consider the capabilities of the display screen and the storage capacity of the billboard's computer system.

There might be a limit to how high the resolution can be.

Great.

Hopefully, you've got answers similar to this.

But if not, maybe you want to rewind the video and look over the slides again.

We're going to move on to the third part of the lesson.

So we've calculated the file size of an image, we've looked at the effects on the different resolutions, and now we are going to have a look at the effects of using different colour depths.

Hopefully you're enjoying this lesson as much as I am.

I'm really looking forward to this last part.

Colour depth refers to the number of bits used to represent the colour of a single pixel in an image or display.

Higher colour depth allows for more colours, leading to smoother gradients and more realistic images.

This image is split in two.

Each half has a different colour depth.

So on the left-hand side, we've got a 24 bit colour depth.

And then on the right-hand side, I've changed the image to have 8 bit colour depth, which only uses 256 colours compared to the 16.

7 million colours on the left.

And hopefully you can see that difference of quality.

Colour depth is typically set to either 8 bits, 256 colours that's normally used in old systems. 16 bit, which is 65,536 colour, so this is high colour used in early multimedia.

Or 24 bits, which gives us 16.

7 million colours.

This is like known as the true colour and its standard now for most modern day images and displays.

In real life, we can perceive a huge amount of colour variation.

Therefore, the more colours you use in a graphic or an image, the closer the picture will look to reality.

So high colour depths result in richer and more accurate colour.

However, more bits per pixel mean larger file sizes because each pixel requires more data to store its colour information.

So here, we have a picture of a hot air balloon that I've divided into four sections.

The section on the left-hand side will require more data than the section on the right-hand side because it uses more colours.

You can see how much the file size decreases when the colour depth is decreased in these images.

So 24 bits of colour on the left-hand side gives us those 16 million colours, which uses 2.

3 megabytes.

The next one is eight bits, So it gives us 256 colours and needs 23.

4 kilobytes to be stored.

The third one along is an 8 bit colour.

It has 256 different types of greys, which is stored or needs to have 23.

4 kilobytes of storage.

So that's the same as the one that we just seen, but it's just different types of greys.

And this last one just uses two colours, white and black.

So therefore, you need 2.

9 kilobytes to store this image.

The reason you may choose to reduce the colour depth is to reduce the file size.

The more bits used to represent colours in an image, the more storage spaces needed to save that image.

Sending smaller file size images via mobile phone messages allows them to be received more quickly and uses less of the senders data allowance.

So for example, 8 bits could be a simple graphics, icons or retro style games.

16 bits of colour depth may be used for basic multimedia or older games.

Or 24 or 32 bits colour depth will be used for photography or video editing and high quality displays.

Let's have a practise.

So "Sam is creating an image to promote her school's charity event.

She has two versions of the same image.

Image A has 414,948 different colours.

And image B has 256 different colours.

One, which image will have the smallest file size and why?" You to have a think about it? You can pause the video at any time or rewind it.

Two, "If someone wants to use the image on the school's website, which image would be more suitable and why?" Three, "Which image would be best for printing a large high quality banner poster to be displayed on the school's gates?" And so you can say why.

Remember, you can rewind the video at any time or you can pause it.

Let's have a look at the answer.

So we have these two different images here.

One, which image will be the smallest file size and why? So it'll be image B.

That will have the smallest file size 'cause it means it's got a lower colour depth, which means the image takes up less storage space.

Well done if you've got the answer correct.

Two, "If someone wants to use the image on the school's website, which image would be more suitable and why? Image B would be more suitable for the website.

Smaller file sizes are important for websites so they load up quickly.

An image with a high colour depth would make the web page load slowly.

And three, "Which image would be best for printing a large high quality banner poster to be displayed on the school's gates and why?" So it would be image A 'cause it's got a high colour depth.

It would be best for the poster.

Images with a high colour depth have more detail and will look clearer when printed in a large size.

So well done if you've got those answers correct.

Or if you haven't, maybe you want to rewind the video to remind yourselves of the answers.

Well done.

You've come to the end of the lesson.

You've done so well.

I've really enjoyed learning with you today.

I hope you've enjoyed it too.

So over the lesson, you've that resolution refers to the number of pixels in an image.

Higher resolution means more pixels and a more detailed image.

You've also learned that colour depth refers to the number of bits used to represent the colour of each pixel.

Higher colour depth means a wider range of colours.

And also you've learned there's a balance between image quality and file size.

Higher quality images, more pixels, more colours, usually have larger file sizes.

So you've learned a huge amount.

So, well done.

I'm really pleased.

Thank you.