Browser image formats:
- GIF
- JPG (or JPEG)
- a few others less commonly used (e.g. PNG)
- all are raster formats, not vector
we will consider only GIF and JPG
- indexes colours.
- makes histogram of colours in image, selects 256 most common colours, discards the rest.
- pixels with other colours are converted to closest remaining colour.
- Most of our maps have only a few colours.
the JPG process:
- software breaks image into small squares.
- each square is analyzed to find pattern of colour variation.
- mathematical model is found which resembles that pattern.
- smaller squares or closer match gives good image but not much compression.
- larger squares or weaker match gives poor image but great compression.
- very poor results around sharp edges.
- very good results with gradients.
- good for photos, bad for most drawings.
Our maps are vector drawings.
- convert vector to raster.
- place drawing on an imaginary grid.
- each grid cell becomes one pixel in raster image.
- each grid cell crossed by a line is given the line colour.
- this includes all lettering.
- only effective if the grid cells are small enough.
- small grid cells capture fine detail.
- but many cells make large image.
- trade-off between detail and file size.
- converting other drawings may cause problems:
- low resolution, text or fine lines hard to read.
- high resolution, legible but file too big.
pointers for web drawings:
- use larger fonts, so still clear after conversion.
- use thicker lines (not hairline).
- or omit line (use transparent line around coloured shape)
- avoid gradient shading.
Some images are combinations of drawing and photo.
- export as GIF and JPG, see which works best.
- use maximum quality settings on JPG for best results.
- avoid gradients if using GIF.
Exporting Corel drawings as GIF or JPG:
- use file - export menu, not 'save as...'
GIF and JPG images can be opened in Photoshop.
- but remember you can't edit objects separately.
options:
- change size.
- change brightness, colours etc.
- add photos or other details.
- you may prefer to add photos to drawing at this stage.
limitations:
- many effects can't be used with indexed images.
- other effects can be used but don't work as well.
- change indexed colour to RGB first, in Photoshop.