A display controller to upscale a source video image for display on a display unit of a computer system. An encoder circuit stores in a local memory pixel data of previous scan lines required for interpolation in a compressed format using differential pulse code modulation (DPCM) scheme. As a part of the DPCM scheme, encoder generates a predicted value for each source video pixel data as a function of at least one prior pixel data value in the scan line. However, when a slope over load condition is encountered, the decoder circuit changes the predicted value for a subsequent pixel data value to a different value, which may enable the graphics controller circuit to avoid a slope overload condition as to subsequent pixel data values. A decoder circuit decompresses the pixel data into original format prior to sending to an interpolator. The interpolator receives a present scan line and the decompressed data of previous scan lines, and interpolates the received pixels to generate additional pixels required for upscaling the source video image. When a slope overload condition is encountered, a override circuitry causes the interpolator to use for interpolation a pixel data value in the present scan line instead of the pixel data value causing the slope overload condition.
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