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Computer Graphics Bitmaps & Sprites

Learn about the basics of bitmap graphics, file formats, sprites, blending methods, transparency, and alpha data in computer graphics.

coryanderson
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Computer Graphics Bitmaps & Sprites

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  1. Computer GraphicsBitmaps & Sprites CO2409 Computer Graphics Week 3

  2. Today’s Lecture • Bitmaps & File Formats • Sprites • Blending Methods • Transparency / Alpha Data • Alpha Blending

  3. Limitations of 2D Geometry • Rendering 2D geometry is simplistic, but can be effective for: • Line drawings • Vector-based work • Simple graphic design • But to render anything complex would need excessive detail • Use another method…

  4. Bitmaps • A bitmap is rectangle of pixels stored off-screen • Bitmaps can come from several sources: • Hand-drawn (e.g. using Photoshop, Maya or even last week’s PixelPlotter) • A single capture from a camera, scanner or video • Other 2D data, e.g. satellite data • An algorithm for generating pixels (procedural textures)

  5. Displaying Bitmaps • The pixels of a bitmap are usually copied into a rectangular region of a viewport • Can be the entire viewport (a background) • If region copied to is same size as the bitmap this is a simple pixel-by-pixel copy • Otherwise the bitmap will must be scaled • Will see details this operation when we look at textures • We can also copy a bitmap to a rotated or distorted region – a more complex process • These operations can be done in hardware

  6. Bitmap File Formats • We usually need to store bitmaps for reuse • There are many possible data formats, each with different benefits and drawbacks: • BMP (Windows bitmap) • JPEG (.jpg) (Joint Photographic Experts Group) • GIF (Graphics Interchange Format) • PNG (Portable Network Graphics) • TGA (Truevision Targa) • DDS (DirectDraw Surface – bitmap for DirectX) • And many more… • One of the example bitmaps earlier would not typically be stored in a file, which one?

  7. General Bitmap Format • Most bitmap formats contain additional data apart from the pixels: • Width and Height • Colour space/range (e.g. RGB 0-255) • Other data can be stored: • E.g. printing size, title, comments, pixel format etc. • Pixel data may be stored in a range of formats: • Raw pixel data (R, G & B values): TGA, BMP • Compressed: GIF (zip), TGA,BMP (RLE) • Lossy Compressed: JPEG

  8. Bitmap Compression • Pixel data can be compressed in most formats • Some compression methods only store an approximation of the pixels for better compression • Called lossy compression • The idea is that the eye does not notice the inaccuracies • Jpeg’s are a notable example • Repeatedly saving in a lossy format will degrade quality • Lossy compression schemes can reduce data size 100x or more PNG format (Non-Lossy Compressed) JPEG (Lossy)

  9. Sprites • A sprite is a name given to a particular use of bitmaps • Sprites are used as distinct elements in a larger scene, e.g: • A character in a 2D game • A mouse cursor • A letter in a font • Sprites may be layered (perhaps with a background image) to create a complex picture. • Example is from Metal Slug 3

  10. Pixel Data for Sprites • Sprites usually represent non-rectangular objects • Cannot just copy data from bitmap to viewport as described above • The bitmap rectangle will obscure any detail below • Need to overlay the sprite with the background in some way: • Using cutout or blending methods

  11. Pixel Blending • Pixel blending are methods to combine the RGB colours in the bitmap with those already on the viewport • We use a blending equation to precisely specify the method used • Different blending equations give different effects • Blending creates various forms of transparency: • Semi-transparent objects: glass, plastic, membranes • Effects that brighten the scene: fire, glow, lens flare • Or obscure (darken) the scene: smoke, dust, shadows • Plus other effects: e.g. user interface elements

  12. Multiplicative Blending • The multiplicative blending equation is: Final.Red = Bitmap.Red * Viewport.Red Similar for green and blue, RGB range is [0-1] • Combine bitmap pixel colour with existing viewport pixel colour • Final colour the new colour written to the viewport • Do seperately for R, G & B • If RGB in range 0-255, result would need to be divided by 255 • This is a darkening effect, suitable for representation of glass, shadows, smoke etc • If source pixel is white, destination colour stays same • As source pixel tends to black, destination is darkened

  13. Multiplicative Blending Example Test Pattern Multiplicative Smoke

  14. Additive Blending and Others • The additive blending equation is: Final.Red = Bitmap.Red + Viewport.Red Similar for green and blue, RGB range is [0-1] or [0-255] • This is a lightening effect, mainly used for the representation of lights • Bitmap black = no effect, brightens as bitmap becomes whiter • Many other blending equations are possible, e.g.: Final.Red = 2 * (Bitmap.Red * Viewport.Red) • Darkens & lightens depending on bitmap colour Final.Red = (Bitmap.Red + Viewport.Red) / 2 • Average of bitmap and viewport colour

  15. Additive Blending Example Test Pattern Additive Flare

  16. Transparency / Alpha Data • The previous examples used RGB data only • For each pixel can store an extra value to identify if the pixel is transparent or not • Can be a single bit: • 1: visible, 0: transparent • Or sometimes the other way round • This is called Alpha data, or the alpha channel • Using this data we can draw a cutout sprite: • Only copy the pixels that have a non-zero alpha value

  17. Blending using Alpha • We can specify a range of alpha values for a pixel (not just 0 or 1) • Allowing us to set a level of transparency for the pixel • Allows semi-transparent pixels • Alpha values are usually in the range 0-255 or 0.0-1.0 and are called A • Treated just like R, G and B • Packed together with RGB to make RGBA colours • Can use this alpha channel in blending methods • Usually only the bitmap alpha value is used • Generally avoid use of viewport alpha

  18. Alpha Blending • The usual alpha blending equation is: Final.Red = Bitmap.Red * Bitmap.Alpha + Destination.Red * (1 – Bitmap.Alpha) Similar for green and blue, RGBA range is [0-1] • [N.B. This formula is a linear interpolation between bitmap and viewport colour – we will see this formula again] • Notice how only the bitmap alpha is used • Alpha blending allows for variable transparency based on the alpha value • Low alpha = very transparent, high alpha = very opaque • It can be used for precise transparency effects

  19. Alpha Blending Example Test Pattern RGB Channel Alpha allows variable transparency Alpha Channel

  20. Packed Pixel Data with Alpha • So we have now identified four channels of data for a typical pixel: • Red, Green, Blue and Alpha • Such data is often called ARGB or RGBA data • If each channel is a byte (0-255) then a single pixel is a double-word (4 bytes) • Efficient for hardware, called ARGB 8888 format • There are other packing schemes: • 1 bit for A, 5 bits for R,G & B: ARGB 1555 format • 4 bits each for A, R, G & B: ARGB 4444 format • 5 bits for R & B, 6 for G, 0 for A: RGB 565 • 32 bit floats for each: ARGB 32F32F32F32F • These will be relevant for GPU-based graphics later…

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