You can draw a new form with copybits and use your own color mapping. First, I thought there would be a bitblt operation that you could use without explicit calculating the color mapping, a paint operation that would convert a gray-value to an alpha value. But I could find it.
In this special case (black text on white background) you can use an existing method for calculating this color map. This method is used by strike font to create apropriate glyph forms from its font bitmap. (A large bitmap with black characters on a white background.) Here is an example that first, creates a form with black text on white background, and than computes the color map and copys the source form in a new form: "create a test form " sourceForm := Form extent: 300@150 depth: 32. sourceForm fillColor: Color white. "draw some black, bold text" text := 'Hello World' asText. text addAttribute: (TextFontReference toFont: (LogicalFont familyName: 'Source Sans Pro' pointSize: 32)). text addAttribute: (TextColor color: Color black). text addAttribute: (TextEmphasis bold). text asMorph drawOn: sourceForm getCanvas. "create a morph for displaying this form" sourceImageMorph := ImageMorph withForm: sourceForm. sourceImageMorph openInWorld; topLeft: 300@300. "calculate the colormap" colorMap := Color computeColorConvertingMap: Color black from: 32 to: 32 keepSubPixelAA: false. "copy the source form bits to a new form of the same size" newForm := sourceForm deepCopy copyBits: sourceForm boundingBox from: sourceForm at: 0@0 colorMap: (colorMap). "create a morph for displaying this form" newImageMorph := ImageMorph withForm: newForm. newImageMorph openInWorld; topLeft: 300@500. 2017-12-27 22:43 GMT+01:00 Andy Burnett <andy.burn...@knowinnovation.com>: > I have an image with black text, and a white background. I would like to > set the white pixels to transparent. I worked out how to do this, but of > course it leaves me with a white halo around the text, where the border > pixels are almost white, but not quite. > > In googling about this, I discovered that RMagick has the idea of > replacing pixels which are almost any particular colour. I didn't find > anything similar in the ColorForm class. Is there a way to do this within > Pharo? > > Cheers > Andy >
